DP Chemistry (first assessment 2025)

Question 21M.2.SL.TZ1.2

Date	May 2021	Marks available	[Maximum mark: 8]	Reference code	21M.2.SL.TZ1.2
Level	SL	Paper	2	Time zone	TZ1
Command term	Calculate, Demonstrate, Determine, Draw, Predict, State	Question number	2	Adapted from	N/A

[Maximum mark: 8]

21M.2.SL.TZ1.2

Iron (II) sulfide reacts with hydrochloric acid to form hydrogen sulfide, H₂S.

In aqueous solution, hydrogen sulfide acts as an acid.

(a(i))

Draw the Lewis (electron dot) structure of hydrogen sulfide.

[1]

Markscheme

OR ✔

Accept any combination of lines, dots or crosses to represent electrons.

(a(ii))

Predict the shape of the hydrogen sulfide molecule.

[1]

Markscheme

bent/non-linear/angular/v-shaped✔

(b(i))

State the formula of its conjugate base.

[1]

Markscheme

HS⁻ ✔

(b(ii))

Saturated aqueous hydrogen sulfide has a concentration of 0.10 mol dm⁻³ and a pH of 4.0. Demonstrate whether it is a strong or weak acid.

[1]

Markscheme

weak AND strong acid of this concentration/[H⁺] = 0.1 mol dm⁻³ would have pH = 1
OR
weak AND [H⁺] = 10⁻⁴ < 0.1 «therefore only fraction of acid dissociated» ✔

(b(iii))

Calculate the hydroxide ion concentration in saturated aqueous hydrogen sulfide.

[1]

Markscheme

10⁻¹⁰ «mol dm⁻³» ✔

(c)

A gaseous sample of nitrogen, contaminated only with hydrogen sulfide, was reacted with excess sodium hydroxide solution at constant temperature. The volume of the gas changed from 550 cm³ to 525 cm³.

Determine the mole percentage of hydrogen sulfide in the sample, stating one assumption you made.

[3]

Markscheme

Mole percentage H₂S:
volume of H₂S = «550 − 525 = » 25 «cm³» ✔
mol % H₂S = « $\frac{25 {cm}^{3}}{550 {cm}^{3}} \times 100$ = » 4.5 «%» ✔

Award [2] for correct final answer of 4.5 «%»

Assumption:
«both» gases behave as ideal gases ✔

Accept “volume of gas $α$ mol of gas”.
Accept “reaction goes to completion”.
Accept “nitrogen is insoluble/does not react with NaOH/only H₂S reacts with NaOH”.

Syllabus sections

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.1—A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons. Deduce the Lewis formula of molecules and ions for up to four electron pairs on each atom.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.4—The valence shell electron pair repulsion (VSEPR) model enables the shapes of molecules to be predicted from the repulsion of electron domains around a central atom. Predict the electron domain geometry and the molecular geometry for species with up to four electron domains.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.2—A pair of species differing by a single proton is called a conjugate acid–base pair. Deduce the formula of the conjugate acid or base of any Br.nsted–Lowry base or acid.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.4—The pH scale can be used to describe the [H+] of a solution: pH = –log10[H+]; [H+] = 10–pH. Perform calculations involving the logarithmic relationship between pH and [H+].

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.6—Strong and weak acids and bases differ in the extent of ionization. Recognize that acid–base equilibria lie in the direction of the weaker conjugate. and are strong acids, and group 1 hydroxides are strong bases.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.5—The ion product constant of water, Kw, shows an inverse relationship between [H+] and [OH–]. Kw = [H+] [OH–]. Recognize solutions as acidic, neutral and basic from the relative values of [H+] and [OH–].

Structure 1. Models of the particulate nature of matter » Structure 1.5—Ideal gases » Structure 1.5.3—The molar volume of an ideal gas is a constant at a specific temperature and pressure. Investigate the relationship between temperature, pressure and volume for a fixed mass of an ideal gas and analyse graphs relating these variables.

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