IB Chemistry HL 1.4 The Mole Questions

Practise IB Chemistry HL 1.4 by linking formulae to particle counts, calculating solution concentrations and interpreting gas-volume ratios in reactions.

Exam points

Convert between mass, amount and specified particle counts using n=m/M and Avogadro’s constant.
Calculate molar mass in g mol⁻¹ from a formula, including hydrated salts.
Define Aᵣ against carbon-12, then calculate Mᵣ or percentage composition from a formula.
Derive empirical formulae from composition data and molecular formulae using Mᵣ.
Calculate c=n/V with correct cm³–dm³ conversion, then solve dilution and mixing problems.
Use balanced-equation mole ratios and titration data to find unknown amounts or concentrations.
Apply Avogadro’s law and balanced gas ratios to calculate volumes at stated conditions.

Covered syllabus points

1.4.1—Mole (mol)1.4.2—Relative atomic mass (Ar) and relative formula mass (Mr)1.4.3—Molar mass (M)1.4.4—Empirical and molecular formulas 1.4.5—Molar concentration 1.4.6—Avogadro's law

[Maximum number: 1]

How much ethanol contains $1.20 \times 10^{24}$ atoms of carbon?

Avogadro's constant, L or $N_{\mathrm{A}}: 6.02 \times 10^{23} \mathrm{~mol}^{-1}$

$0.333 \mathrm{~mol}$

$0.500 \mathrm{~mol}$

$1.00 \mathrm{~mol}$

$2.00 \mathrm{~mol}$

[Maximum number: 4]

Two groups of students (Group A and Group B) carried out a project* on the chemistry of some group 7 elements (the halogens) and their compounds.

(a)

The students reacted ICl(l) with CsBr(s) to form a yellow solid, $\mathrm{CsICl}_{2}(\mathrm{~s})$ , as one of the products. $\mathrm{CsICl}_{2}(\mathrm{~s})$ has been found to produce very pure CsCl(s) which is used in cancer treatment.

To confirm the composition of the yellow solid, Group A determined the amount of iodine in 0.2015 g of $\mathrm{CsICl}_{2}(\mathrm{~s})$ by titrating it with $0.0500 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})$ . The following data were recorded for the titration.

Table for Question (a) — IB Chemistry HL

[ 4 ]

(i)

Calculate the percentage of iodine by mass in $\mathrm{CsICl}_{2}(\mathrm{~s})$ , correct to three significant figures.

[ 1 ]

$\left(\frac{126.90}{330.71} \times 100\right)=38.4 \%$ ;

(ii)

Determine the amount, in mol, of $0.0500 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})$ added in the titration.

[ 1 ]

$\left(\frac{24.20 \times 5.00 \times 10^{-2}}{1000}\right)=1.21 \times 10^{-3} / 0.00121(\mathrm{~mol})$ ;

(iii)

Calculate the mass of iodine, in g , present in the sample of $\mathrm{CsICl}_{2}(\mathrm{~s})$ .

[ 1 ]

$\quad\left(126.90 \times 6.05 \times 10^{-4}\right)=7.68 \times 10^{-2} / 0.0768(\mathrm{~g})$ ;

(iv)

Determine the percentage by mass of iodine in the sample of $\mathrm{CsICl}_{2}(\mathrm{~s})$ , correct to three significant figures, using your answer from (v).

[ 1 ]

$\left(\frac{7.68 \times 10^{-2}}{0.2015} \times 100\right)=38.1 \%$ ;

[Maximum number: 6]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

(a)

2.478 g of white phosphorus was used to make phosphine according to the equation:

\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})

[ 2 ]

(i)

Calculate the amount, in mol, of white phosphorus used.

[ 1 ]

« $\left\langle\frac{2.478}{4 \times 30.97}\right\rangle$ » =0.02000 «mol»

(ii)

Determine the volume of phosphine, measured in $\mathrm{cm}^{3}$ at standard temperature and pressure, that was produced.

[ 1 ]

(b)

Impurities cause phosphine to ignite spontaneously in air to form an oxide of phosphorus and water.

[ 4 ]

(i)

The oxide formed in the reaction with air contains 43.6% phosphorus by mass. Determine the empirical formula of the oxide, showing your method.

[ 3 ]

$n(\mathrm{P})=\frac{43.6}{30.97}=1.41\ \mathrm{mol}$ $n(\mathrm{O})=\frac{100-43.6}{16.00}=3.53\ \mathrm{mol}$ $\frac{n(\mathrm{O})}{n(\mathrm{P})}=\frac{3.53}{1.41}=2.50$

Empirical formula:
$\mathrm{P}_{2}\mathrm{O}_{5}$

Accept other methods where the working is shown.

(ii)

The molar mass of the oxide is approximately $285 \mathrm{~g} \mathrm{~mol}^{-1}$ .

Determine the molecular formula of the oxide.

[ 1 ]

« $\frac{285}{141.9}=2.00$ , so molecular formula $=2 \times \mathrm{P}_{2} \mathrm{O}_{5}=$ » $\mathrm{P}_{4} \mathrm{O}_{10}$

[Maximum number: 1]

Two groups of students (Group A and Group B) carried out a project* on the chemistry of some group 7 elements (the halogens) and their compounds.

Calculate the percentage of iodine by mass in $\mathrm{CsICl}_{2}(\mathrm{~s})$ , correct to three significant figures.

$\left(\frac{126.90}{330.71} \times 100\right)=38.4 \%$ ;

[Maximum number: 1]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

2.478 g of white phosphorus was used to make phosphine according to the equation:

\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})

Calculate the amount, in mol, of white phosphorus used.

« $\left\langle\frac{2.478}{4 \times 30.97}\right\rangle$ » =0.02000 «mol»

[Maximum number: 1]

Phenylethene (styrene) is produced from ethylbenzene in a gas-phase equilibrium.

Benzoic acid is a weak acid. The graph shows how the pH changes during the titration of a $10.0 \mathrm{~cm}^{3}$ aqueous solution of benzoic acid with aqueous sodium hydroxide.

Calculate the concentration of the benzoic acid solution, in $\mathrm{mol} \mathrm{dm}^{-3}$ , given that the sodium hydroxide concentration was $0.010 \mathrm{~mol} \mathrm{dm}^{-3}$ .

$\text { « }(0.020 \times 0.010) \div 0.010=\text { » }$

0.020 «mol dm ${ }^{-3}$ »

[Maximum number: 1]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

2.478 g of white phosphorus was used to make phosphine according to the equation:

\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})

Determine the volume of phosphine, measured in $\mathrm{cm}^{3}$ at standard temperature and pressure, that was produced.

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Covered syllabus points

Question 1

Question 2

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question 3

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)

Question (ii)

Question 4

Question 5

Question 6

Question 7