IB Chemistry HL S1.4 Counting particles by mass: the mole questions

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Type
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Question 1

Paper 2Medium8 marks

A quality-control test estimates the hydrogen peroxide concentration in a solution labelled 10.0 g dm⁻³. A 10.00 cm³ sample is completely decomposed according to the equation:

2 H₂O₂(aq) → 2 H₂O(l) + O₂(g)

The oxygen produced has a volume of 36.0 cm³. A 25.0 cm³ sample of nitrogen at the same temperature and pressure contains 1.00 × 10⁻³ mol of nitrogen. Assume both gases behave ideally and that no oxygen is lost. Use M(H₂O₂) = 34.0 g mol⁻¹.

(a)

Determine the amount, in mol, of oxygen produced.

[2]
(b)

Calculate the molar concentration, [H₂O₂], in the original solution, in mol dm⁻³.

[3]
(c)

Calculate the mass concentration of hydrogen peroxide in the original solution, in g dm⁻³.

[1]
(d)

Using the labelled concentration as the reference, calculate the percentage by which your result in (c) differs from the label, and state whether the measured value is higher or lower.

[2]

Question 2

Multiple choiceHard1 mark

Equal masses of iron and glucose are compared. Count particles as Fe atoms in iron and C₆H₁₂O₆ molecules in glucose. Which row correctly compares the numbers of particles and the total numbers of atoms?

ElementCHOFe
Ar12.011.0116.0055.85
Show answer and explanation

Answer: D. For equal sample masses, particle number is inversely proportional to relative particle mass. Mr(C₆H₁₂O₆) = 6(12.01) + 12(1.01) + 6(16.00) = 180.18 (no units), which is greater than Ar(Fe) = 55.85; therefore, the iron sample has more particles. Each glucose molecule contains 24 atoms, so total atom numbers are proportional to 24/180.18 for glucose and 1/55.85 for iron. The glucose sample has more atoms overall.

A: treats relative mass as directly proportional to particle number; its conclusion about total atoms is correct.
B: gets the particle comparison right but overlooks the 24 atoms in each glucose molecule.
C: uses direct proportionality for particle number and fails to account for the 24 atoms per glucose molecule when comparing total atoms.
Remember: equal masses contain more particles of the substance with the lower relative particle mass, but total atom counts also depend on atoms per particle.

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