These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Data for every question: H 1.008, C 12.01, N 14.01, O 16.00, Na 22.99, Cl 35.45, K 39.10, Ca 40.08, Al 26.98, S 32.06 g mol⁻¹; N_A = 6.022 × 10²³ mol⁻¹.
Question 1 (multiple choice · foundation)
What is the molar mass of calcium nitrate, Ca(NO₃)₂?
- (A) 102.09 g mol⁻¹
- (B) 132.10 g mol⁻¹
- (C) 150.09 g mol⁻¹
- (D) 164.10 g mol⁻¹
Answer and explanation
Answer: (D). The bracket doubles everything inside: 1 Ca, 2 N and 6 O. M = 40.08 + 2(14.01) + 6(16.00) = 40.08 + 28.02 + 96.00 = 164.10 g mol⁻¹.
- (A) treats the formula as CaNO₃ (ignores the subscript 2).
- (B) counts only 4 oxygen atoms, as if the formula were Ca(NO₂)₂.
- (C) doubles the oxygen but not the nitrogen.
Question 2 (multiple choice · core)
Which sample contains the greatest number of molecules?
- (A) 1.0 g of H₂
- (B) 10.0 g of H₂O
- (C) 16.0 g of O₂
- (D) 22.0 g of CO₂
Answer and explanation
Answer: (B). The sample with the most moles has the most molecules, so compare n = m / M.
| Sample | M (g mol⁻¹) | n (mol) |
|---|---|---|
| 1.0 g H₂ | 2.016 | 0.496 |
| 10.0 g H₂O | 18.02 | 0.555 |
| 16.0 g O₂ | 32.00 | 0.500 |
| 22.0 g CO₂ | 44.01 | 0.500 |
(A), (C) and (D) are all close to 0.50 mol, which is the trap: they look equal to (B) at a glance. Only (B) is clearly above 0.50 mol. Choosing (D) because it has the largest mass confuses mass with number of particles.
Question 3 (multiple choice · core)
What is the average mass of one carbon atom?
- (A) 1.99 × 10⁻²³ g
- (B) 5.01 × 10²² g
- (C) 12.01 g
- (D) 7.23 × 10²⁴ g
Answer and explanation
Answer: (A). One mole (6.022 × 10²³ atoms) has a mass of 12.01 g, so one atom has a mass of 12.01 g ÷ 6.022 × 10²³ = 1.99 × 10⁻²³ g.
- (B) divides the wrong way round (6.022 × 10²³ ÷ 12.01). Its unit would be atoms per gram, not grams.
- (C) is the mass of a whole mole of carbon atoms.
- (D) multiplies instead of dividing. A single atom cannot weigh more than the Earth’s oceans.
Question 4 (calculation · core)
How many oxygen atoms are in 0.250 mol of aluminium sulfate, Al₂(SO₄)₃?
Worked solution
- Oxygen atoms per formula unit: 3 sulfate groups × 4 O = 12 O atoms.
- Moles of O atoms: 0.250 mol × 12 = 3.00 mol.
- Number of O atoms: 3.00 mol × 6.022 × 10²³ mol⁻¹ = 1.81 × 10²⁴ oxygen atoms.
Suggested mark points (2): 1 for using 12 O per formula unit; 1 for the correct final answer to 3 significant figures. A method that first finds the number of formula units (1.506 × 10²³) and then multiplies by 12 is equally valid.
Common error: using 4 O per formula unit (ignoring the bracket) gives 6.02 × 10²³, which is three times too small.
Question 5 (calculation · core)
A sample of an unknown gas contains molecules with an average mass of 7.31 × 10⁻²³ g each. Which of the following is the gas: CO, CO₂, NO₂ or SO₂? Show your reasoning.
Worked solution
- Molar mass = mass of one molecule × N_A = 7.31 × 10⁻²³ g × 6.022 × 10²³ mol⁻¹ = 44.0 g mol⁻¹.
- Compare: CO 28.01, CO₂ 44.01, NO₂ 46.01, SO₂ 64.06 g mol⁻¹.
- The gas is CO₂.
Suggested mark points (2): 1 for multiplying by N_A to get about 44.0 g mol⁻¹; 1 for identifying CO₂ with the comparison shown. NO₂ (46.01) is close but outside the precision of the data, so it is not accepted.
Question 6 (constructed response · core)
A student has two vials. Vial X holds 5.00 g of sodium chloride, NaCl. Vial Y holds 5.00 g of potassium chloride, KCl. The student claims: “The vials contain the same number of formula units because the masses are equal.”
(a) Calculate the amount, in moles, of each compound. (b) Evaluate the student’s claim, using your answers to (a). (c) Calculate the mass of KCl that would contain the same number of formula units as vial X.
Worked solution and suggested Marlbridge rubric
(a) M(NaCl) = 22.99 + 35.45 = 58.44 g mol⁻¹, so n = 5.00 ÷ 58.44 = 0.0856 mol. M(KCl) = 39.10 + 35.45 = 74.55 g mol⁻¹, so n = 5.00 ÷ 74.55 = 0.0671 mol.
(b) The claim is incorrect. Equal masses contain equal numbers of formula units only if the molar masses are equal. Each K⁺ ion is heavier than an Na⁺ ion, so 5.00 g of KCl holds fewer formula units: 0.0671 mol (4.04 × 10²² formula units) against 0.0856 mol (5.15 × 10²² formula units) in vial X.
(c) Same number of formula units means same amount in moles: m = 0.08556 mol × 74.55 g mol⁻¹ = 6.38 g of KCl.
| Point | What earns it |
|---|---|
| 1 | Correct moles of NaCl with working |
| 1 | Correct moles of KCl with working |
| 1 | States the claim is wrong and links it to the different molar masses (or different particle masses) |
| 1 | 6.38 g of KCl, using the moles of NaCl from (a) |
Accept answers from (a) carried forward into (c). Do not award the point in (b) for “the claim is wrong” without a reason.
Question 7 (explanation · stretch)
The average mass of one atom of an element, in amu, has the same number as the mass of one mole of that element, in grams. Explain why this makes it possible to “count” atoms by weighing a sample, and state one assumption you make when you do this.
Model answer and suggested Marlbridge rubric
Model answer. A balance measures mass, not numbers of atoms. Because one mole of any element has a mass in grams equal to the average atomic mass in amu, the molar mass tells us how many grams contain exactly 6.022 × 10²³ atoms. Dividing the measured mass by the molar mass therefore gives the amount in moles, and multiplying by Avogadro’s number gives the number of atoms, without counting them individually.
Assumption (any one): the sample is pure (all the mass is that element); the sample has the natural mix of isotopes that the average atomic mass is based on; the balance reading is accurate.
| Point | What earns it |
|---|---|
| 1 | Links the measured mass to moles through the molar mass, with Avogadro’s number giving the particle count |
| 1 | A valid assumption, such as purity or natural isotopic abundance |
How did you do?
- Q1 or Q4 wrong: re-read the bracket rule in the study guide.
- Q2, Q3 or Q5 wrong: practise the “always go through moles” route in Figure 1 of the study guide.
- Q6 or Q7 incomplete: your reasoning needs the why (molar masses differ), not only the numbers.
Then tick off the topic checklist.