IB Chemistry HL Structure 1: Models of the particulate nature of matter questions

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Question 1

Paper 2Medium6 marks

A laboratory has several batches of a dry solid containing crystalline sulfur and sodium chloride. The relative amounts of the two substances vary between batches. Sodium chloride dissolves in water, whereas sulfur does not. A student wants to obtain dry sodium chloride from one batch.

(a)

Classify each of sulfur and sodium chloride as an element, compound or mixture.

[2]

(b)

State one feature, other than the variable proportions, that distinguishes this sample from a compound.

[1]

(c)

Outline a sequence of steps to obtain dry sodium chloride from the sample.

[3]

Question 2

Multiple choiceMedium1 mark

A copper ion is represented by $^{65}_{29}\mathrm{Cu}^{2+}$ What are its numbers of protons, neutrons and electrons, respectively?

Show answer and explanation

Answer: A. For a nuclide, the proton number is $Z$ and the neutron number is $A-Z$ a $2+$ charge means two fewer electrons than protons. Thus $p=29$ $n=65-29=36$ and $e=29-2=27$

B: adds 2 to the proton number when finding electrons, reversing the charge effect.
C: uses the electron count instead of the proton number in $A-Z$ giving 38 neutrons.
D: assumes the ion has the same number of electrons as a neutral copper atom, ignoring its charge.

Remember: ion formation changes the number of electrons, not the nucleus.

Question 3

Paper 2Medium10 marks

A representative sample of natural boron is characterized for use in a neutron-absorbing material. It contains the isotopes ¹⁰₅B and ¹¹₅B. The relative isotopic masses and relative atomic mass of the sample are:

Isotope	Relative isotopic mass
¹⁰₅B	10.0129
¹¹₅B	11.0093

The relative atomic mass, Aᵣ, of the sample is 10.811.

(a)

For the ion ¹⁰₅B⁺, state the number of protons, neutrons and electrons.

[3]

(b)

Explain why ¹⁰₅B and ¹¹₅B are isotopes.

[2]

(c)

Assuming the sample contains only these two isotopes, determine the percentage abundance of ¹⁰₅B. Give your answer to three significant figures.

[3]

(d)

A student claims that ¹⁰₅B and ¹¹₅B must have identical physical properties because they have the same number of protons. Explain why this claim is not justified.

[2]

Question 4

Multiple choiceMedium1 mark

Three groups of lines in hydrogen’s emission spectrum converge at different photon energies. For the group whose lines converge at the highest photon energy, which energy level do the transitions end at, and what is the maximum number of electrons that level can hold?

Show answer and explanation

Answer: A. The highest-energy convergence limit corresponds to transitions ending at the lowest energy level, $n=1$ Its maximum capacity is $2n^2=2(1)^2=2$ electrons.
B: assigns the limit to $n=2$ then correctly applies $2n^2$
C: assigns the limit to $n=3$ then correctly applies $2n^2$
D: identifies $n=1$ but omits the factor of 2 in the capacity relation.
Remember: the maximum number of electrons in level $n$ is $2n^2$

Question 5

Paper 2Medium7 marks

The wavelength corresponding to the limit of convergence at higher frequency in the emission spectrum of gaseous element Q is $285.7\ \mathrm{nm}$

A database gives the following successive ionization energies for Q.

Ionization energy	Value / $\mathrm{kJ\,mol^{-1}}$
First	419
Second	3052
Third	4420
Fourth	5877

(a)

Calculate the first ionization energy of Q in $\mathrm{kJ\,mol^{-1}}$ using the data booklet. Give your answer to three significant figures.

[3]

(b)

Use your result from (a) and the successive ionization energy data.

(i)

State whether your result from (a) agrees with the database value for the first ionization energy to three significant figures.

[1]

(ii)

Deduce the group of Q, explaining your deduction using the successive ionization energy data.

[3]

Question 6

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 7

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?

Element	C	H	O	Fe
Ar	12.01	1.01	16.00	55.85

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.

Question 8

Multiple choiceMedium1 mark

A fixed amount of an ideal gas has pressure $8.0 \times 10^4\ \mathrm{Pa}$ volume $6.0 \times 10^{-3}\ \mathrm{m^3}$ and temperature $300\ \mathrm{K}$ It is changed to volume $3.0 \times 10^{-3}\ \mathrm{m^3}$ and temperature $450\ \mathrm{K}$ What is its final pressure?

Show answer and explanation

Answer: D. For a fixed amount of gas, $P \propto T/V$ Thus, $P_2=P_1(V_1/V_2)(T_2/T_1)=(8.0\times10^4)(2.0)(1.5)=2.4\times10^5\ \mathrm{Pa}$

A: reverses the temperature ratio, using $T_1/T_2$ instead of $T_2/T_1$
B: accounts for the temperature increase but omits the volume decrease.
C: accounts for the volume decrease but omits the temperature increase.
Remember: at fixed amount, pressure increases with temperature and decreases with volume.

Question 9

Paper 2Medium6 marks

A chemist compares equal amounts of gaseous nitrogen at two different sets of conditions. Figure 1 shows schematic particle diagrams for the samples.

(a)

State three assumptions of the ideal-gas model.

[3]

(b)

Using Figure 1, identify the sample expected to show the greater deviation from ideal-gas behaviour.

[1]

(c)

Using Figure 1 and your knowledge of the ideal-gas model, explain how the conditions for this sample favour deviation from ideal-gas behaviour.

[2]

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