Question

A mixture containing $N{a}_{2}C{O}_3,NaOH$ and inert matter weights $0.75$gm. When the aqueous solution is titrated with $0.5NHCl$, the colour of phenolphthalein disappears when $21.00$ ml of the acid has been added. Methyl range is then added and $7.00$ ml more of the acid is required to give red colour to the solution. The % of $N{a}_{2}C{O}_3$ is ________.

• A. $49.5$%
• B. $24.5$%
• C. $37.1$%
• D. none of the above

Answer 1

The correct option is C $37.1$%

Let Meq of $NaOH=x$ and Meq of $N{a}_{2}C{O}_3=y$
Meq of $NaOH+\frac{1}{2}MeqN{a}_{2}C{O}_3=MeqHCl$

$x+\frac{y}{2}=0.5\times 21=10.5$ ----- (1)

$\frac{MeqN{a}_{2}C{O}_3}{2}$= Meq $HCl$

$\frac{y}{2}=0.5\times 7$

$⟹y=7$

Now $\frac{N{a}_{2}C{O}_3}{\frac{106}{2}}\times 1000=y$

$N{a}_{2}C{O}_3=0.371$