Question

$21.2\text{g}$ sample of impure $N{a}_{2}C{O}_3$ is dissolved and reacted with a solution of $CaC{l}_2$, the weight of precipitate of $CaC{O}_3$ is $10.0\text{g}$. Which of the following statements is/are correct ?

• A. The $\%$ purity of $N{a}_{2}C{O}_3$ is $50\%$
• B. The percentage purity of $N{a}_{2}C{O}_3$ is $60\%$
• C. The number of moles of $N{a}_{2}C{O}_3=$ that of $CaC{O}_3=0.1\text{mol}$
• D. The number of moles of $NaCl$ formed is $0.1\text{mol}$

Answer 1

Answer: (A), (C)

The correct options are
A The $\%$ purity of $N{a}_{2}C{O}_3$ is $50\%$
C The number of moles of $N{a}_{2}C{O}_3=$ that of $CaC{O}_3=0.1\text{mol}$

Molar mass of $CaC{O}_3=100{\text{g mol}}^{-1}$
Molar mass of $N{a}_{2}C{O}_3=106{\text{g mol}}^{-1}$

$N{a}_{2}C{O}_3+CaC{l}_2\to CaC{O}_3+2NaCl$
(a) From stoichiometry,
moles of $CaC{O}_3=\frac{10}{100}=0.1\text{mol}$
moles of $N{a}_{2}C{O}_3=$ moles of $CaC{O}_3=\frac{1}{2}\times$ moles of $NaCl$
mass of $N{a}_{2}C{O}_3=0.1\times 106=10.6\text{g}$
$\%$ purity $N{a}_{2}C{O}_3=\frac{10.6}{21.2}=100=50\%$

c) moles of $CaC{O}_3=\frac{10}{100}=0.1\text{mol}$
(d) Moles of $NaCl=2\times 0.1=0.2\text{mol}$