CAIE A-Level Chemistry 2 Atoms, Molecules and Stoichiometry

Practise moving between relative masses, particles, moles, formulae, equations and reacting quantities.

Covered syllabus points

2.1 Relative masses of atoms and molecules 2.2 The mole and the Avogadro constant 2.3 Formulas 2.4 Reacting masses and volumes (of solutions and gases)

[Maximum number: 1]

Calcium, magnesium and radium are Group 2 elements. Radium follows the same trends as the other members of Group 2.

A sample of magnesium contains three isotopes, ${ }^{25} \mathrm{Mg},{ }^{26} \mathrm{Mg}$ and X.

The percentage abundance of the three isotopes is shown in Table 1.1.

Table 1.1

The relative atomic mass, $A_{r}$ , is calculated by comparing the average mass of the isotopes of an element to the unified atomic mass unit.

Define the unified atomic mass unit.

1 / 12 (one twelfth) the mass of a carbon- $12 /{ }^{12} \mathrm{C}$ atom

[Maximum number: 2]

Chlorine exists as a diatomic gas, $\mathrm{Cl}_{2}(\mathrm{~g})$ . A sample of $\mathrm{Cl}_{2}(\mathrm{~g})$ was made during a chemical reaction. When measured at 404 kPa and $25^{\circ} \mathrm{C}$ the sample occupied a volume of $20.0 \mathrm{~cm}^{3}$ .

Calculate the number of chlorine atoms in this sample of $\mathrm{Cl}_{2}(\mathrm{~g})$ . You may find it helpful to use your answer to (d)(i).

If you are unable to calculate an answer to (d)(i), use 0.36 g of $\mathrm{Cl}_{2}$ . This is not the correct answer.

number of chlorine atoms =

Method 1 & Method 2

$\mathbf{M} \mathbf{1}=3.263 \times 10^{-3} \times 2$ & $\mathbf{M} \mathbf{1}=\frac{0.23}{71.0} \times 2$ OR $6.53 \times 10^{-3}$ $\mathbf{M} \mathbf{2}=6.02 \times 10^{23} \times \mathbf{M} \mathbf{1}$ $=3.93 \times 10^{21}$ atoms of Cl

$\mathbf{M} \mathbf{2}=6.02 \times 10^{23} \times \mathbf{M} \mathbf{1}$ $=3.90 \times 10^{21}$ atoms of Cl

[Maximum number: 5]

Species such as $\mathrm{NH}_{4}{ }^{+}, \mathrm{CO}_{3}{ }^{2-}$ and $\mathrm{PO}_{4}{ }^{3-}$ are examples of molecular ions.

(a)

$\mathrm{NH}_{4}{ }^{+}$ is a Brønsted-Lowry acid.

[ 1 ]

(i)

When $\mathrm{NH}_{4}{ }^{+}(\mathrm{aq})$ is heated with NaOH(aq), a pungent gas is produced.

Write an ionic equation for this reaction.

[ 1 ]

ionic equation $\quad \mathrm{NH}_{4}{ }^{+}+\mathrm{OH}^{-} \rightarrow \mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O}$

(b)

There are many naturally occurring hydrated compounds that contain the anion $\mathrm{PO}_{4}{ }^{3-}$ .

[ 4 ]

(i)

Name the anion $\mathrm{PO}_{4}{ }^{3-}$ .

[ 1 ]

(ii)

Struvite is a soft hydrated mineral with $M_{\mathrm{r}}=245.3$ . The anhydrous form of the mineral has the formula $\mathrm{NH}_{4} \mathrm{MgPO}_{4}$ .

Calculate the number of molecules of water of crystallisation in struvite.
Give your answer to the nearest integer. Show your working.

number of molecules of water of crystallisation =

[ 3 ]

(molecular mass of $\mathrm{NH}_{4} \mathrm{MgPO}_{4}$ ) = 137.3

$\left(245.3-M_{r}\left(\mathrm{NH}_{4} \mathrm{MgPO}_{4}\right) \div 18.0\right)$ =6

[Maximum number: 3]

Bismuth is an element in Group 15 of the Periodic Table.

$\mathrm{NaBiO}_{3}$ can be used to determine the concentration of $\mathrm{Mn}^{2+}(\mathrm{aq})$ . The ionic equation for the reaction is shown in equation 2 .

$equation $2 \quad 2 \mathrm{Mn}^{2+}+5 \mathrm{BiO}_{3}^{-}+14 \mathrm{H}^{+} \rightarrow 2 \mathrm{MnO}_{4}^{-}+5 \mathrm{Bi}^{3+}+7 \mathrm{H}_{2} \mathrm{O}$$

A student uses the following procedure in an experiment.
- Add $100.0 \mathrm{~cm}^{3}$ of a saturated solution of $\mathrm{Mn}^{2+}(\mathrm{aq})$ to a volumetric flask.
- Add distilled water to the flask to make a $1.00 \mathrm{dm}^{3}$ diluted solution.
- Titrate a $25.00 \mathrm{~cm}^{3}$ sample of the diluted solution with $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{NaBiO}_{3}(\mathrm{aq})$ .

The $25.00 \mathrm{~cm}^{3}$ sample of the diluted solution of $\mathrm{Mn}^{2+}(\mathrm{aq})$ reacts completely with exactly $21.50 \mathrm{~cm}^{3}$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{NaBiO}_{3}(\mathrm{aq})$ .

Calculate the concentration, in $\mathrm{moldm}^{-3}$ , of $\mathrm{Mn}^{2+}(\mathrm{aq})$ in the saturated solution.
Show your working.

concentration of $\mathrm{Mn}^{2+}(\mathrm{aq})$ in the saturated solution = $\mathrm{mol} \mathrm{dm}^{-3}$

moles of $\mathrm{NaBiO}_{3}=21.50 / 1000 \times 0.100 \quad\left(=2.15 \times 10^{-3} \mathrm{~mol}\right)$

moles of $\mathrm{Mn}^{2+}$ in dil. $=2.15 \times 10^{-3} \times 2 / 5 \times 40(=0.0344 \mathrm{~mol})$

conc. saturated $\mathrm{Mn}^{2+}=0.0344 \times 1000 / 100=\mathbf{0 . 3 4 4 ~ m o l ~ d m}{ }^{-3}$

All question bank results loaded

Covered syllabus points

2. Atoms, molecules and stoichiometry question 1

2. Atoms, molecules and stoichiometry question 2

2. Atoms, molecules and stoichiometry question 3

Question (a)

Question (i)

Question (b)

Question (i)

Question (ii)

2. Atoms, molecules and stoichiometry question 4