Question #44664

A $1.325 \text{ g}$ sample of impure anhydrous sodium carbonate ($\ce{Na2CO3}$) is dissolved in water and treated with $50.0 \text{ mL}$ of $0.50 \text{ M}$ $\ce{HCl}$. The solution is boiled to completely expel dissolved $\ce{CO2}$. The excess acid is then back-titrated with $0.20 \text{ M}$ aqueous ammonia ($\ce{NH3}$). The equivalence point of the back-titration is reached after adding $25.0 \text{ mL}$ of the $\ce{NH3}$ solution.

Given: $K_b(\ce{NH3}) = 2.0 \times 10^{-5}$, $K_w = 1.0 \times 10^{-14}$, $\log 5 = 0.70$. The total volume of the solution at the equivalence point is exactly $100.0 \text{ mL}$.

Determine the percentage purity of the $\ce{Na2CO3}$ sample and identify the most suitable indicator for the back-titration from the following options: Methyl Red ($pK_{In} = 5.1$), Bromothymol Blue ($pK_{In} = 7.1$), Phenolphthalein ($pK_{In} = 9.4$).