A 3.25\text{ g} sample of impure zinc granules reacts completely with excess dilute tetrao… — JAMB UTME

3.25\text{ g}

sample of impure zinc granules reacts completely with excess dilute tetraoxosulfate(VI) acid according to the equation:

\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}

896\text{ cm}^3

of hydrogen gas is collected at STP, calculate the percentage purity of the zinc sample.

[\text{Zn} = 65\text{; Molar volume of gas at STP} = 22.4\text{ dm}^3\text{mol}^{-1}]

Answer: 80 %

80%

The volume of hydrogen gas produced at STP (

896\text{ cm}^3 = 0.896\text{ dm}^3

) corresponds to

0.04\text{ mol}

\text{H}_2

. Based on the

1:1

stoichiometric reaction between zinc and tetraoxosulfate(VI) acid,

0.04\text{ mol}

of pure zinc was present in the sample. The mass of pure zinc is

0.04\text{ mol} \times 65\text{ g/mol} = 2.60\text{ g}

. Dividing this pure mass by the total sample mass of

3.25\text{ g}

and multiplying by

100\%

yields a percentage purity of

80\%

Convert the collected volume of hydrogen gas from cm³ to dm³

\text{Volume of H}_2 = \frac{896\text{ cm}^3}{1000\text{ cm}^3/\text{dm}^3} = 0.896\text{ dm}^3

Molar gas volume is given in dm³/mol, so gas volume must be expressed in dm³.

Calculate the amount in moles of hydrogen gas produced at STP

\text{Moles of H}_2 = \frac{0.896\text{ dm}^3}{22.4\text{ dm}^3\text{mol}^{-1}} = 0.04\text{ mol}

Number of moles of gas at STP equals volume divided by molar volume.

Determine the moles and mass of pure zinc that reacted

\text{Moles of pure Zn} = 0.04\text{ mol}

;

\text{Mass of pure Zn} = 0.04\text{ mol} \times 65\text{ g/mol} = 2.60\text{ g}

The reaction stoichiometry shows a 1:1 mole ratio between Zn and H₂.

Calculate the percentage purity of the zinc sample

\text{Percentage Purity} = \left( \frac{2.60\text{ g}}{3.25\text{ g}} \right) \times 100\% = 80\%

Percentage purity is the ratio of pure substance mass to total sample mass expressed as a percentage.

Percentage purity determination via gas volume stoichiometry at STP

Question