Question

$0.848g$ aqueous solution of a mixture containing $N{a}_{2}C{O}_3,NaOH$, and an inert matter is titrated with $M/2$ $HCl$. The colour of phenolphthalein disappears when $20mL$ of the acid has been added. Methyl orange is then added and $0.8mL$ more of the acid is required to give a red colour to the solution. The percentage of $N{a}_{2}C{O}_3$ is

• A. 25
• B. $12.5$
• C. 75
• D. 50

Answer 1

Answer: (D)

Let $x$ and $y$ are the $mEq$ of $NaOH$ and $N{a}_{2}C{O}_3$ respectively
Phenolphthalein as indicator:
$mEq$ of $NaOH=\frac{1}{2}mEq$ of $N{a}_{2}C{O}_3=mEq$ of $HCl$
$x+\frac{y}{2}=\frac{1}{2}\times 1\times 20=10$...(i)
Methyl orange as indicator:
$\frac{mEqN{a}_{2}C{O}_3(left)}{2}=mEq$ of $HCl$
$\frac{y}{2}=\frac{1}{2}\times 8=\mathrm{4....}$(ii)
$y=8,x=6$
Weight of $N{a}_{2}C{O}_3=8\times 10^{-3}\times \frac{106}{2}=0.424g$
$\%$ of $N{a}_{2}C{O}_3=\frac{0.424}{0.848}\times 100=5\%$