A 4.20\text{ g} sample of impure sodium hydrogentrioxocarbonate(IV), \text{NaHCO}_3, was t… — JAMB UTME

4.20\text{ g}

sample of impure sodium hydrogentrioxocarbonate(IV),

\text{NaHCO}_3

, was thermally decomposed according to the equation:

2\text{NaHCO}_3\text{(s)} \rightarrow \text{Na}_2\text{CO}_3\text{(s)} + \text{H}_2\text{O(g)} + \text{CO}_2\text{(g)}

0.448\text{ dm}^3

of carbon(IV) oxide gas was collected at s.t.p., what is the percentage purity of the

\text{NaHCO}_3

sample?

[\text{Na} = 23, \text{H} = 1, \text{C} = 12, \text{O} = 16, \text{molar volume of gas at s.t.p.} = 22.4\text{ dm}^3\text{ mol}^{-1}]

Answer: 80 %

The percentage purity of the sodium hydrogentrioxocarbonate(IV) sample is 80.0%.

The volume of CO₂ gas collected (

0.448\text{ dm}^3

) corresponds to

0.020\text{ mol}

at s.t.p. According to the balanced equation,

2\text{ mol}

\text{NaHCO}_3

decompose to form

1\text{ mol}

\text{CO}_2

, meaning

0.040\text{ mol}

of pure

\text{NaHCO}_3

reacted. Multiplying by the molar mass of

\text{NaHCO}_3

(

84\text{ g mol}^{-1}

) gives

3.36\text{ g}

of pure compound, which represents

80.0\%

of the original

4.20\text{ g}

sample.

Calculate the moles of carbon(IV) oxide gas evolved at s.t.p.

0.020 mol of CO₂

Dividing the volume of gas collected by the molar volume of a gas at s.t.p. gives the chemical amount in moles.

Determine the moles of pure NaHCO₃ using the mole ratio from the balanced chemical equation.

0.040 mol of NaHCO₃

The reaction stoichiometry shows a 2:1 mole ratio between NaHCO₃ and CO₂.

Calculate the mass of pure NaHCO₃ by multiplying its moles by its molar mass (84 g/mol).

3.36 g of pure NaHCO₃

Mass is obtained by converting moles to grams using molar mass.

Divide the mass of pure NaHCO₃ by the initial mass of the impure sample (4.20 g) and multiply by 100%.

80.0%

Percentage purity expresses the proportion of active compound relative to the total mass of the sample.

Percentage purity calculation based on gas stoichiometry

Estimated Time:1m 30s

Question