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  4. When 100 mL of 80 % (w/v) NaOH is mixed with certain amount of 40 %( w / w ) NaOH solution of density 1.2 g / mL, the percentage concentration of final solution becomes 60 % (w/v). Find the molarity of the final solution.

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Q. When $100 \,mL$ of $80 \%$ (w/v) $NaOH$ is mixed with certain amount of $40 \%( w / w ) NaOH$ solution of density $1.2\, g / mL$, the percentage concentration of final solution becomes $60 \%$ (w/v). Find the molarity of the final solution.

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Solution:

$0.6=\frac{80+V \times 1.2 \times 0.4}{100+V}$
$60+0.6 V=80+0.48 V$
$0.12 V=20$
$V=\frac{1000}{6} mL$
Total mass of $NaOH$ mixed $=80+V \times 1.2 \times 0.4$
$=80+\frac{1000}{6} \times 1.2 \times 0.4=160 \,g . $
Molarity $=15 \,M$