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  4. 0.848 g aqueous solution of a mixture containing Na 2 CO 3, NaOH, and an inert matter is titrated with M / 2 HCl. The colour of phenolphthalein disappears when 20 mL of the acid has been added. Methyl orange is then added and 0.8 mL more of the acid is required to give a red colour to the solution. The percentage of Na 2 CO 3 is

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Q. $0.848\, g$ aqueous solution of a mixture containing $Na _2 CO _3, NaOH$, and an inert matter is titrated with $M / 2$ $HCl$. The colour of phenolphthalein disappears when $20 \,mL$ of the acid has been added. Methyl orange is then added and $0.8 \,mL$ more of the acid is required to give a red colour to the solution. The percentage of $Na _2 CO _3$ is

Some Basic Concepts of Chemistry

Solution:

Let $x$ and $y$ are the $mEq$ of $NaOH$ and $Na _2 CO _3$ respectively
Phenolphthalein as indicator:
$mEq$ of $NaOH =\frac{1}{2} mEq$ of $Na _2 CO _3=m E q$ of $HCl$
$x+\frac{y}{2}=\frac{1}{2} \times 1 \times 20=10$...(i)
Methyl orange as indicator:
$\frac{ mEq \,Na _2 CO _3( left )}{2}= mEq$ of $HCl$
$\frac{y}{2}=\frac{1}{2} \times 8=4 .... $(ii)
$y=8, x=6$
Weight of $Na _2 CO _3=8 \times 10^{-3} \times \frac{106}{2}=0.424\, g$
$\%$ of $Na _2 CO _3=\frac{0.424}{0.848} \times 100=5 \%$