Chapter 3 - "Stoichiometry" - Short Questions
"Stoichiometry" - Short Questions
1. What is meant by stoichiometry?
Answer:
Stoichiometry is the area of chemistry that deals with calculating the quantities of reactants and products involved in chemical reactions.
2. Which law is the basis of stoichiometry?
Answer:
The law of conservation of mass, which states that mass is neither created nor destroyed in a chemical reaction.
3. What does a chemical formula show?
Answer:
A chemical formula shows the elements present in a substance and the number of atoms of each element.
4. Why must chemical equations be balanced?
Answer:
Chemical equations must be balanced to show that the same number of each type of atom is present before and after the reaction.
5. What information does a word equation provide?
Answer:
A word equation shows the names of the reactants and products involved in a chemical reaction.
6. What is the purpose of a symbol equation?
Answer:
A symbol equation uses chemical formulae to show the substances involved and the ratios in which they react.
7. What are state symbols?
Answer:
State symbols show the physical state of a substance in a reaction, such as solid (s), liquid (l), gas (g), or aqueous (aq).
8. Why do ionic compounds have no overall charge?
Answer:
Ionic compounds have no overall charge because the total positive charge equals the total negative charge.
9. What is meant by a spectator ion?
Answer:
A spectator ion is an ion that does not take part in a chemical reaction and remains unchanged.
10. Why is stoichiometry important in chemistry?
Answer:
Stoichiometry is important because it allows chemists to calculate amounts of substances and predict the outcomes of chemical reactions accurately.
11. What is meant by a chemical formula?
Answer:
A chemical formula is a symbolic representation that shows which elements are present in a substance and the number of atoms of each element.
12. Why are chemical symbols used in formulae?
Answer:
Chemical symbols are used because they are internationally agreed and allow chemists around the world to communicate clearly and accurately.
13. What information does a chemical formula give at the particle level?
Answer:
A chemical formula shows the types of atoms present and the ratio in which they are combined in a substance.
14. Why is it important to learn the formulae of common elements and compounds at IGCSE level?
Answer:
It is important because chemical formulae are needed to write equations, predict reactions, and carry out chemical calculations.
15. Do chemical formulae describe the physical appearance of a substance?
Answer:
No, chemical formulae do not describe appearance. They only show the composition of a substance, which helps explain its properties and reactions.
16. What is an element?
Answer:
An element is a pure substance made of only one type of atom.
17. What rules must be followed when writing chemical symbols?
Answer:
The first letter must be capitalized, and if there is a second letter, it must be written in lowercase.
18. Give two examples of elements that exist as single atoms.
Answer:
Sodium (Na) and neon (Ne) exist as single atoms.
19. What is meant by a diatomic element?
Answer:
A diatomic element is an element that exists naturally as molecules made of two atoms bonded together.
20. Why must oxygen be written as O₂ and not O in equations?
Answer:
Oxygen exists as a diatomic molecule, so O₂ shows that two oxygen atoms are bonded together in one molecule.
21. What is a compound?
Answer:
A compound is a pure substance formed when two or more different elements are chemically combined in fixed proportions.
22. What does the chemical formula of a compound show?
Answer:
The chemical formula shows the types of atoms present and the number of each type in the smallest unit of the compound.
23. Give the chemical formula of water and explain what it shows.
Answer:
The formula of water is H₂O. It shows that each molecule contains two hydrogen atoms and one oxygen atom.
24. Why is the formula of calcium chloride written as CaCl₂?
Answer:
Calcium chloride is written as CaCl₂ because one calcium ion combines with two chloride ions to form a neutral compound.
25. What does a subscript in a chemical formula represent?
Answer:
A subscript shows the number of atoms or ions of the element immediately before it.
26. What is meant by the molecular formula of a compound?
Answer:
The molecular formula shows the type and number of atoms present in one molecule of a compound.
27. What type of bonding is found in compounds that have molecular formulae?
Answer:
Compounds with molecular formulae are held together by covalent bonds.
28. What two pieces of information does a molecular formula provide?
Answer:
It shows which elements are present and how many atoms of each element are in one molecule.
29. What does the molecular formula H₂O tell you about a water molecule?
Answer:
It shows that one molecule of water contains two hydrogen atoms and one oxygen atom.
30. Why does the formula CO represent a different substance from CO₂?
Answer:
CO contains one carbon atom and one oxygen atom, while CO₂ contains one carbon atom and two oxygen atoms, giving them different compositions and properties.
31. What does a subscript apply to in a molecular formula?
Answer:
A subscript applies only to the element immediately before it.
32. Why is correct capitalization important in molecular formulae?
Answer:
Because different capitalization can represent different substances, for example CO is carbon monoxide while Co is cobalt.
33. How does a molecular formula differ from an empirical formula?
Answer:
A molecular formula shows the actual number of atoms in a molecule, while an empirical formula shows the simplest whole-number ratio of atoms.
34. Why do all molecules of a compound have the same molecular formula?
Answer:
Because compounds have a fixed composition, meaning the atoms are always combined in the same ratio.
35. Why are molecular formulae important when writing chemical equations?
Answer:
They are needed to balance equations correctly and ensure the law of conservation of mass is obeyed.
36. What information is used to deduce the formula of a compound from a diagram?
Answer:
The relative numbers of each type of atom shown in the diagram are used to deduce the formula.
37. In a particle diagram, what does each circle or sphere represent?
Answer:
Each circle or sphere represents a single atom.
38. What is meant by the term “relative numbers of atoms”?
Answer:
It refers to the ratio in which different atoms are present compared to one another.
39. A diagram shows two hydrogen atoms for every one oxygen atom. What is the formula?
Answer:
The formula is H₂O.
40. Why is a formula written using the simplest whole-number ratio?
Answer:
Because chemical formulae always represent the simplest ratio of atoms in a compound.
41. A diagram shows four hydrogen atoms and two oxygen atoms bonded together. Why is the formula not H₄O₂?
Answer:
Because the ratio of hydrogen to oxygen is 2:1, so the simplest formula is H₂O.
42. A molecule contains one carbon atom bonded to four hydrogen atoms. What is its formula?
Answer:
The formula is CH₄.
43. How do you identify different elements in a diagram?
Answer:
Different elements are shown using different colours, sizes, or labels.
44. A diagram shows one nitrogen atom surrounded by three hydrogen atoms. What is the formula?
Answer:
The formula is NH₃.
45. A repeating diagram shows one magnesium atom for every two chlorine atoms. What is the formula?
Answer:
The formula is MgCl₂.
46. What is a chemical reaction?
Answer:
A chemical reaction is a process in which reactants are changed into new substances called products.
47. What does a chemical equation show?
Answer:
A chemical equation shows the reactants, the products, and how they are related in a chemical reaction.
48. What is a word equation?
Answer:
A word equation uses the names of substances to show a chemical reaction.
49. Write the word equation for magnesium reacting with oxygen.
Answer:
Magnesium + oxygen → magnesium oxide
50. What is a symbol equation?
Answer:
A symbol equation uses chemical symbols and formulae to represent a chemical reaction.
51. Why must symbol equations be balanced?
Answer:
Because atoms are not created or destroyed in a chemical reaction, so the number of each type of atom must be the same on both sides.
52. What is the meaning of a coefficient in a chemical equation?
Answer:
A coefficient shows how many molecules or formula units of a substance take part in the reaction.
53. What do the state symbols (s), (l), (g), and (aq) represent?
Answer:
(s) solid, (l) liquid, (g) gas, and (aq) aqueous solution.
54. Write the symbol equation for zinc reacting with hydrochloric acid.
Answer:
Zn + 2HCl → ZnCl₂ + H₂
55. Why are state symbols important in chemical equations?
Answer:
They show the physical states of substances and help explain observations such as gas formation or precipitates.
56. What is meant by the empirical formula of a compound?
Answer:
The empirical formula is the simplest whole-number ratio of the different atoms or ions present in a compound.
57. How does an empirical formula differ from a molecular formula?
Answer:
An empirical formula shows the simplest ratio of atoms, while a molecular formula shows the actual number of atoms in one molecule.
58. Why does an empirical formula always use whole numbers?
Answer:
Because atoms and ions are discrete particles and cannot exist in fractional amounts.
59. What is the empirical formula of a compound with the molecular formula C₆H₁₂O₆?
Answer:
The empirical formula is CH₂O.
60. Why are empirical formulae especially important for ionic compounds?
Answer:
Because ionic compounds do not exist as molecules and their formulae show the simplest ratio of ions in the lattice.
61. What is the empirical formula of sodium chloride?
Answer:
The empirical formula is NaCl.
62. A compound contains magnesium and chlorine in the ratio 1:2. What is its empirical formula?
Answer:
The empirical formula is MgCl₂.
63. If a diagram shows 2 carbon atoms and 4 hydrogen atoms repeatedly, what is the empirical formula?
Answer:
The empirical formula is CH₂.
64. Why is the empirical formula of water the same as its molecular formula?
Answer:
Because the ratio of hydrogen to oxygen atoms is already in the simplest form, 2:1.
65. What law of chemistry is reflected by the empirical formula?
Answer:
The law of definite proportions, which states that a compound always contains the same elements in the same ratios.
66. What is an ionic compound?
Answer:
An ionic compound is a substance formed from positive and negative ions held together by electrostatic attraction.
67. Why do ionic compounds not exist as individual molecules?
Answer:
Because the ions form a giant ionic lattice with repeating patterns, not separate molecules.
68. What does the formula of an ionic compound represent?
Answer:
It represents the simplest whole-number ratio of ions present in the lattice.
69. What must always be true about the total charge in an ionic compound?
Answer:
The total positive charge must equal the total negative charge so the compound is neutral.
70. How can a diagram be used to deduce an ionic formula?
Answer:
By counting the relative numbers of each type of ion and writing the simplest ratio.
71. A diagram shows one magnesium ion and two chloride ions. What is the formula?
Answer:
The formula is MgCl₂.
72. How is the formula of an ionic compound deduced using ion charges?
Answer:
By balancing the charges so that the total positive and negative charges cancel out.
73. What is the formula of calcium oxide formed from Ca²⁺ and O²⁻ ions?
Answer:
The formula is CaO.
74. Why is the formula of aluminium oxide Al₂O₃?
Answer:
Because two Al³⁺ ions and three O²⁻ ions are needed to balance the charges.
75. Why are ionic formulae described as empirical formulae?
Answer:
Because they show the simplest ratio of ions and not a molecular formula.
76. What is a symbol equation?
Answer:
A symbol equation represents a chemical reaction using chemical symbols and formulae instead of words.
77. What does the arrow (→) in a symbol equation mean?
Answer:
It means “reacts to form” or “produces”.
78. Why must all symbol equations be balanced?
Answer:
Because of the law of conservation of mass, which states that atoms are not created or destroyed in a reaction.
79. Why are coefficients used instead of changing subscripts when balancing equations?
Answer:
Because changing subscripts changes the identity of a substance, while coefficients only change the number of units reacting.
80. Balance the equation:
Mg + O₂ → MgO
Answer:
2Mg + O₂ → 2MgO
81. What information do state symbols provide in a symbol equation?
Answer:
They show the physical state of each substance during the reaction.
82. What does the state symbol (aq) mean?
Answer:
It means the substance is dissolved in water to form an aqueous solution.
83. Write the symbol equation with state symbols for zinc reacting with hydrochloric acid.
Answer:
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
84. What is a spectator ion?
Answer:
A spectator ion is an ion that does not take part in the chemical reaction and remains unchanged.
85. Write the ionic equation for the neutralization of an acid by an alkali.
Answer:
H⁺(aq) + OH⁻(aq) → H₂O(l)
86. What does the term “deduce” mean in chemistry questions?
Answer:
To deduce means to work out the correct answer logically using the information provided.
87. What four things must be identified before writing a symbol equation?
Answer:
The reactants, the products, the correct chemical formulae, and the physical states.
88. Which state symbol is used for a substance dissolved in water?
Answer:
The state symbol is (aq).
89. Bubbles are observed when a metal reacts with an acid. What gas is usually produced?
Answer:
Hydrogen gas is produced.
90. Write the balanced symbol equation with state symbols for magnesium reacting with hydrochloric acid.
Answer:
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
91. What does the state symbol (g) tell you about a product?
Answer:
It shows that the product is a gas.
92. Copper is heated in air and a black solid forms. What is the symbol equation with state symbols?
Answer:
2Cu(s) + O₂(g) → 2CuO(s)
93. What type of reaction occurs when calcium carbonate breaks down on heating?
Answer:
Thermal decomposition.
94. Write the symbol equation with state symbols for the thermal decomposition of calcium carbonate.
Answer:
CaCO₃(s) → CaO(s) + CO₂(g)
95. What does the formation of a solid from two aqueous solutions indicate?
Answer:
It indicates that a precipitate has formed.
96. What is meant by relative atomic mass (Ar)?
Answer:
Relative atomic mass is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12.
97. Why is relative atomic mass used instead of the actual mass of an atom?
Answer:
Because the actual mass of an atom is too small to measure conveniently, so a relative scale is used instead.
98. Which isotope is used as the standard for relative atomic mass?
Answer:
Carbon-12 is used as the standard.
99. What is meant by 1/12th of the mass of carbon-12?
Answer:
It is defined as one atomic mass unit, which is used to compare the masses of other atoms.
100. Why is relative atomic mass often not a whole number?
Answer:
Because it is an average value based on the masses and abundances of different isotopes.
101. What are isotopes?
Answer:
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
102. Why does chlorine have a relative atomic mass of about 35.5?
Answer:
Because it is a mixture of chlorine-35 and chlorine-37 isotopes, and chlorine-35 is more abundant.
103. Where can the relative atomic mass of an element usually be found?
Answer:
On the Periodic Table, usually below the element symbol.
104. Does relative atomic mass represent the mass of one single atom?
Answer:
No, it represents an average value, not the mass of a single atom.
105. State one use of relative atomic mass in chemistry.
Answer:
It is used to calculate relative molecular mass and reacting masses in chemical equations.
106. What is meant by relative molecular mass (Mr)?
Answer:
Relative molecular mass is the sum of the relative atomic masses of all the atoms present in one molecule of a substance.
107. Why does relative molecular mass have no units?
Answer:
Because it is a relative value that compares masses to 1/12th of a carbon-12 atom, not an actual measured mass.
108. How is the relative molecular mass of water calculated?
Answer:
Water is H₂O, so Mr = (2 × 1) + (1 × 16) = 18.
109. What is the relative molecular mass of carbon dioxide, CO₂?
Answer:
Mr = (1 × 12) + (2 × 16) = 44.
110. What is meant by relative formula mass (Mr)?
Answer:
Relative formula mass is the sum of the relative atomic masses of all the atoms shown in the chemical formula of a substance.
111. Why is relative formula mass used for ionic compounds?
Answer:
Because ionic compounds do not exist as individual molecules and form giant ionic lattices.
112. What is the relative formula mass of sodium chloride, NaCl?
Answer:
Mr = 23 + 35.5 = 58.5.
113. Calculate the relative formula mass of magnesium oxide, MgO.
Answer:
Mr = 24 + 16 = 40.
114. What is the relative formula mass of aluminium oxide, Al₂O₃?
Answer:
Mr = (2 × 27) + (3 × 16) = 102.
115. State one use of Mr in chemistry.
Answer:
It is used to calculate reacting masses or convert between mass and amount of substance.
116. What are reacting masses?
Answer:
Reacting masses are the masses of substances that react together according to a balanced chemical equation.
117. Why must a chemical equation be balanced before calculating reacting masses?
Answer:
Because a balanced equation shows the correct ratio in which substances react by mass.
118. Which values are used to calculate reacting masses at Core level?
Answer:
Relative atomic mass (Ar) and relative molecular or formula mass (Mr).
119. What mass of oxygen reacts with 12 g of magnesium?
Reaction: 2Mg + O₂ → 2MgO
Answer:
From the ratio 3 g Mg : 2 g O,
12 g Mg reacts with 8 g of oxygen.
120. In the reaction 2H₂ + O₂ → 2H₂O, what is the mass ratio of hydrogen to oxygen?
Answer:
The mass ratio is 1 : 8.
121. What mass of oxygen is needed to react with 6 g of carbon to form carbon dioxide?
Reaction: C + O₂ → CO₂
Answer:
12 g of carbon reacts with 32 g of oxygen,
so 6 g of carbon reacts with 16 g of oxygen.
122. What information does the mass ratio in a reaction represent?
Answer:
It shows the fixed proportion by mass in which reactants combine.
123. What mass of hydrochloric acid reacts with 13 g of zinc?
Reaction: Zn + 2HCl → ZnCl₂ + H₂
Answer:
65 g of zinc reacts with 73 g of HCl,
so 13 g of zinc reacts with 14.6 g of HCl.
124. What masses of products are formed from 100 g of calcium carbonate?
Reaction: CaCO₃ → CaO + CO₂
Answer:
56 g of calcium oxide and 44 g of carbon dioxide are formed.
125. Why is the mole concept not needed for these calculations at Core level?
Answer:
Because reacting masses can be calculated directly using relative masses and simple proportions from the balanced equation.
126. What is meant by the concentration of a solution?
Answer:
Concentration is the amount of solute dissolved in a fixed volume of solution.
127. What is the difference between a concentrated and a dilute solution?
Answer:
A concentrated solution contains a large amount of solute in a small volume, while a dilute solution contains a small amount of solute in a large volume.
128. State two units used to measure concentration in IGCSE Chemistry.
Answer:
Grams per cubic decimetre (g / dm³) and moles per cubic decimetre (mol / dm³).
129. What volume does 1 dm³ represent?
Answer:
1 dm³ is equal to 1 litre or 1000 cm³.
130. What does a concentration of 10 g / dm³ mean?
Answer:
It means 10 grams of solute are dissolved in 1 dm³ of solution.
131. Calculate the concentration in g / dm³ if 25 g of sugar is dissolved in 2 dm³ of solution.
Answer:
25 ÷ 2 = 12.5 g / dm³.
132. What does a concentration of 1 mol / dm³ mean?
Answer:
It means 1 mole of solute particles is dissolved in 1 dm³ of solution.
133. Why is mol / dm³ often preferred in chemical calculations?
Answer:
Because chemical reactions depend on the number of particles, not their mass.
134. How does dilution affect the concentration of a solution?
Answer:
Dilution decreases the concentration because the volume increases while the amount of solute stays the same.
135. Which unit of concentration is more suitable for titration experiments?
Answer:
Mol / dm³ is more suitable for titration experiments.
136. What is meant by the mole in chemistry?
Answer:
The mole is the unit of amount of substance and represents a fixed number of particles.
137. What is the value of the Avogadro constant?
Answer:
The Avogadro constant is 6.02 × 10²³ particles per mole.
138. What does the term “amount of substance” refer to?
Answer:
It refers to the number of particles present in a substance.
139. What types of particles can be counted using the mole?
Answer:
Atoms, molecules, ions, formula units, and electrons.
140. How many particles are present in one mole of any substance?
Answer:
One mole contains 6.02 × 10²³ particles.
141. Why is the mole needed in chemistry?
Answer:
Because particles are too small to count individually, and reactions depend on the number of particles.
142. How many molecules are present in one mole of water?
Answer:
One mole of water contains 6.02 × 10²³ water molecules.
143. What is the mass of one mole of carbon atoms?
Answer:
One mole of carbon atoms has a mass of 12 g.
144. Why do different substances have different masses per mole?
Answer:
Because their particles have different relative atomic or molecular masses.
145. How is the mole similar to the term “dozen”?
Answer:
Both are counting units; a dozen means 12 items, while a mole means 6.02 × 10²³ particles.
146. State the formula used to calculate amount of substance.
Answer:
Amount of substance (mol) = mass (g) ÷ molar mass (g / mol)
147. What does molar mass represent?
Answer:
Molar mass is the mass of one mole of a substance and is measured in g / mol.
148. Calculate the amount of substance in 24 g of magnesium.
(Ar of Mg = 24)
Answer:
Amount = 24 ÷ 24 = 1 mol
149. What is the mass of 2 moles of sodium chloride?
(Mr of NaCl = 58.5)
Answer:
Mass = 2 × 58.5 = 117 g
150. How can the formula be rearranged to calculate mass?
Answer:
Mass (g) = amount of substance (mol) × molar mass (g / mol)
151. A substance has a mass of 10 g and an amount of 0.25 mol. What is its molar mass?
Answer:
Molar mass = 10 ÷ 0.25 = 40 g / mol
152. What is the molar mass of water, H₂O?
Answer:
Molar mass = (2 × 1) + 16 = 18 g / mol
153. How many particles are present in 1 mole of a substance?
Answer:
1 mole contains 6.02 × 10²³ particles.
154. How many molecules are present in 0.5 mol of carbon dioxide?
Answer:
0.5 × 6.02 × 10²³ = 3.01 × 10²³ molecules
155. Why must units be checked carefully in mole calculations?
Answer:
Because mass must be in grams, molar mass in g / mol, and amount in moles for the formula to work correctly.
156. What is meant by r.t.p.?
Answer:
r.t.p. means room temperature and pressure, taken as about 20 °C and 1 atmosphere (about 101 kPa).
157. What is the molar gas volume at r.t.p.?
Answer:
One mole of any gas occupies 24 dm³ at r.t.p.
158. Why do all gases have the same molar volume at r.t.p.?
Answer:
Because equal numbers of gas particles occupy equal volumes at the same temperature and pressure.
159. State the formula used to calculate the volume of a gas at r.t.p.
Answer:
Volume (dm³) = amount of substance (mol) × 24
160. What volume does 2 moles of carbon dioxide occupy at r.t.p.?
Answer:
2 × 24 = 48 dm³
161. How is the amount of gas calculated from its volume at r.t.p.?
Answer:
Amount (mol) = volume (dm³) ÷ 24
162. A gas occupies 12 dm³ at r.t.p. How many moles of gas are present?
Answer:
12 ÷ 24 = 0.5 mol
163. How many cubic centimetres are there in 24 dm³?
Answer:
24 dm³ = 24 000 cm³
164. In the reaction 2H₂(g) + O₂(g) → 2H₂O(g), what volume of oxygen reacts with 48 dm³ of hydrogen at r.t.p.?
Answer:
48 dm³ of hydrogen is 2 moles, so oxygen is 1 mole and occupies 24 dm³.
165. Why can gas volume ratios be taken directly from balanced equations at r.t.p.?
Answer:
Because gas volumes are proportional to mole ratios at the same temperature and pressure.
166. What are stoichiometric reacting masses?
Answer:
They are the masses of reactants and products that react together according to the ratios shown in a balanced chemical equation.
167. Why do substances react in fixed mass ratios?
Answer:
Because atoms are conserved in chemical reactions and combine in fixed ratios based on their formulae.
168. What is meant by a limiting reactant?
Answer:
The limiting reactant is the substance that is used up first and limits the amount of product formed.
169. In the reaction 2Mg + O₂ → 2MgO, what mass of oxygen reacts with 48 g of magnesium?
Answer:
48 g of magnesium reacts with 32 g of oxygen.
170. What volume does 0.5 mol of carbon dioxide occupy at r.t.p.?
Answer:
0.5 × 24 = 12 dm³.
171. What is the molar gas volume at r.t.p.?
Answer:
The molar gas volume at r.t.p. is 24 dm³ per mole.
172. Convert 25 cm³ into dm³.
Answer:
25 cm³ ÷ 1000 = 0.025 dm³.
173. Calculate the concentration in g / dm³ if 10 g of solute is dissolved in 0.5 dm³ of solution.
Answer:
10 ÷ 0.5 = 20 g / dm³.
174. Calculate the concentration in mol / dm³ if 0.25 mol of a substance is dissolved in 0.5 dm³.
Answer:
0.25 ÷ 0.5 = 0.50 mol / dm³.
175. How do you calculate the amount of substance from concentration and volume?
Answer:
Amount (mol) = concentration (mol / dm³) × volume (dm³).
176. What is a titration?
Answer:
A titration is a laboratory method used to find the concentration, amount, or volume of a solution by reacting it with another solution of known concentration.
177. What piece of apparatus is used to deliver one solution accurately in a titration?
Answer:
A burette is used to deliver the solution accurately.
178. Why must a balanced chemical equation be used in titration calculations?
Answer:
Because it shows the mole ratio in which the reactants react.
179. State the formula used to calculate moles from titration data.
Answer:
Amount of substance (mol) = concentration (mol / dm³) × volume (dm³)
180. Why must volumes be converted from cm³ to dm³ in calculations?
Answer:
Because concentration is measured in mol / dm³, so volume must be in dm³.
181. Calculate the number of moles in 20.0 cm³ of a 0.100 mol / dm³ acid.
Answer:
20.0 cm³ = 0.020 dm³
Moles = 0.100 × 0.020 = 0.0020 mol
182. In the reaction HCl + NaOH → NaCl + H₂O, what is the mole ratio of acid to alkali?
Answer:
The mole ratio is 1 : 1.
183. If 0.010 mol of NaOH reacts with sulfuric acid, how many moles of H₂SO₄ react?
Reaction: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Answer:
0.010 ÷ 2 = 0.005 mol of H₂SO₄.
184. How is the concentration of a solution calculated from titration results?
Answer:
Concentration (mol / dm³) = amount of substance (mol) ÷ volume (dm³)
185. What is the end point in a titration?
Answer:
The end point is when the indicator changes colour, showing the reaction is complete.
186. What is meant by the empirical formula of a compound?
Answer:
The empirical formula shows the simplest whole-number ratio of atoms of each element in a compound.
187. What is the difference between an empirical formula and a molecular formula?
Answer:
The empirical formula shows the simplest ratio of atoms, while the molecular formula shows the actual number of atoms in one molecule.
188. What is the first step when calculating an empirical formula from mass data?
Answer:
Convert the mass of each element into moles using relative atomic mass.
189. A compound contains 12 g of carbon and 2 g of hydrogen. What is its empirical formula?
Answer:
Carbon: 12 ÷ 12 = 1 mol
Hydrogen: 2 ÷ 1 = 2 mol
Empirical formula = CH₂
190. How is percentage composition used to find an empirical formula?
Answer:
The percentages are treated as masses in 100 g, converted to moles, and simplified to the simplest ratio.
191. A compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen. What is the empirical formula?
Answer:
C: 40 ÷ 12 ≈ 3.33
H: 6.7 ÷ 1 = 6.7
O: 53.3 ÷ 16 ≈ 3.33
Ratio = 1 : 2 : 1
Empirical formula = CH₂O
192. Why are mole ratios sometimes multiplied by whole numbers?
Answer:
To convert fractional ratios into whole-number ratios.
193. What additional information is needed to calculate the molecular formula?
Answer:
The relative molecular mass (Mr) of the compound.
194. The empirical formula of a compound is CH₂ and its Mr is 42. What is its molecular formula?
Answer:
Empirical formula mass = 14
42 ÷ 14 = 3
Molecular formula = C₃H₆
195. When is the molecular formula the same as the empirical formula?
Answer:
When the empirical formula mass is equal to the relative molecular mass.
196. What is meant by percentage yield?
Answer:
Percentage yield is a measure of how much product is actually obtained compared with the theoretical yield, expressed as a percentage.
197. Write the formula used to calculate percentage yield.
Answer:
Percentage yield = (actual yield ÷ theoretical yield) × 100
198. Why is the actual yield usually lower than the theoretical yield?
Answer:
Because reactions may be incomplete, products may be lost during transfer or purification, or side reactions may occur.
199. A reaction has a theoretical yield of 20 g and an actual yield of 15 g. Calculate the percentage yield.
Answer:
Percentage yield = (15 ÷ 20) × 100 = 75%
200. What is meant by percentage composition by mass?
Answer:
It is the percentage of the total mass of a compound that is contributed by each element.
201. Write the formula for percentage composition by mass.
Answer:
Percentage composition = (mass of element ÷ total mass of compound) × 100
202. Calculate the percentage composition of carbon in carbon dioxide, CO₂.
Answer:
Mr of CO₂ = 12 + (2 × 16) = 44
Percentage of carbon = (12 ÷ 44) × 100 ≈ 27.3%
203. What is meant by percentage purity?
Answer:
Percentage purity shows how much of a sample is made up of the desired pure substance.
204. Write the formula for calculating percentage purity.
Answer:
Percentage purity = (mass of pure substance ÷ mass of impure sample) × 100
205. A 25 g impure sample contains 20 g of pure substance. Calculate the percentage purity.
Answer:
Percentage purity = (20 ÷ 25) × 100 = 80%
Thank You!
Sana Shariq
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