Calculating Masses and Moles
What is the mass of 2 moles of carbon dioxide (CO₂)?
(A) 22 g
(B) 44 g
(C) 88 g
(D) 12 g
**Answer**: C (Mᵣ of CO₂ = 44 → Mass = 2 × 44 = 88 g)
The empirical formula of a compound is CH₂. If its Mᵣ is 28, what is its molecular formula?
(A) CH₂
(B) C₂H₄
(C) C₃H₆
(D) C₄H₈
**Answer**: B (Mᵣ of CH₂ = 14 → 28 ÷ 14 = 2 → C₂H₄)
What is the concentration (in mol/dm³) of a solution with 0.5 moles of NaCl in 250 cm³?
(A) 0.5
(B) 1.0
(C) 2.0
(D) 4.0
**Answer**: C (250 cm³ = 0.25 dm³ → Conc. = 0.5 ÷ 0.25 = 2.0 mol/dm³)
In molten PbBr₂, what forms at the anode?
(A) Pb
(B) Br₂
(C) O₂
(D) H₂
**Answer**: B (Br⁻ ions lose electrons → Br₂ gas).
Calculate the number of moles in 36 g of water (H₂O).
**Answer**:Mᵣ of H₂O = 18 → Moles = 36 ÷ 18 = 2 moles.
A compound contains 40% carbon and 60% oxygen. What is its empirical formula?
**Answer**: C: 40 ÷ 12 ≈ 3.33; O: 60 ÷ 16 ≈ 3.75 → Ratio 1:1 → CO.
Convert 0.2 mol/dm³ of H₂SO₄ to g/dm³ (Mᵣ = 98).
**Answer**: Mass = 0.2 × 98 = 19.6 g/dm³.
Write the half-equation for the reduction of Na⁺ at the cathode.
**Answer**: Na⁺ + e⁻ → Na.
If 4.8 g of magnesium reacts with oxygen, what mass of MgO is produced?
(A) 8 g
(B) 16 g
(C) 24 g
(D) 32 g
**Answer**: A (Balanced equation: 2Mg + O₂ → 2MgO; Moles of Mg = 4.8 ÷ 24 = 0.2 → Mass of MgO = 0.2 × 40 = 8 g)
In an experiment, 5.6 g of iron reacts with oxygen to form 8.0 g of iron oxide. What is the empirical formula?
(A) FeO
(B) Fe₂O₃
(C) Fe₃O₄
(D) FeO₂
**Answer**: A (Mass of O = 8.0 – 5.6 = 2.4 g → Fe: 5.6 ÷ 56 = 0.1; O: 2.4 ÷ 16 = 0.15 → Ratio 2:3 → Fe₂O₃)
What volume of 0.1 mol/dm³ NaOH contains 0.02 moles?
(A) 20 cm³
(B) 200 cm³
(C) 2 dm³
(D) 0.2 dm³
**Answer**: D (Volume = 0.02 ÷ 0.1 = 0.2 dm³ = 200 cm³)
In aqueous NaCl electrolysis, what forms at the cathode?
(A) Na
(B) Cl₂
(C) H₂
(D) O₂
**Answer**: C (H⁺ reduced instead of Na⁺ → H₂ gas).
What is the percentage yield if 50 g of product is obtained from a reaction with a theoretical yield of 80 g?
% yield = (50 ÷ 80) × 100 = 62.5%.
Describe how reduction of copper oxide can determine its empirical formula.
**Answer**: Heat CuO with hydrogen; measure mass loss (oxygen lost) and remaining copper mass to calculate the ratio.
How many moles of HCl are in 50 cm³ of 2 mol/dm³ solution?
**Answer**: Moles = 2 × (50 ÷ 1000) = 0.1.
Why are inert electrodes used in electrolysis?
**Answer**: To prevent them reacting with the products.
In the reaction \(2H₂ + O₂ → 2H₂O\), if 10 g of H₂ reacts, what volume of O₂ (at RTP) is required?
**Answer**: Moles of H₂ = 10 ÷ 2 = 5 → Moles of O₂ needed = 2.5 → Volume = 2.5 × 24 = 60 dm³.
A hydrocarbon has 85.7% carbon and 14.3% hydrogen. If its Mᵣ is 56, find its molecular formula.
**Answer**: C: 85.7 ÷ 12 ≈ 7.14; H: 14.3 ÷ 1 ≈ 14.3 → Ratio 1:2 → CH₂. Mᵣ = 14 → 56 ÷ 14 = 4 → C₄H₈.
Describe how to prepare 500 cm³ of 0.5 mol/dm³ CuSO₄ from solid CuSO₄ (Mᵣ = 160).
**Answer**: Mass needed = 0.5 × 0.5 × 160 = 40 g → Dissolve in water and make up to 500 cm³.
Predict the products of electrolysing aqueous CuSO₄ with inert electrodes.
**Answer**: Cathode: Cu (Cu²⁺ reduced); Anode: O₂ (OH⁻ oxidised).