Question

A 500 mL solution is prepared by dissolving 20 g sodium hydroxide in water. This is mixed in another 200 mL solution of $NaOH$ having molarity 0.2 M. The molarity of resulting solution is:

• A. 0.66 M
• B. 0.77 M
• C. 1.80 M
• D. 1.97 M

Answer 1

Answer: (A)

0.66 M

Total volume= $500+200=700ml$

Mass of $NaOH$ in $0.2M$ solution

$\Rightarrow 0.2=\frac{x/40}{200/100}$

$\Rightarrow x=\frac{0.2\times 200\times 40}{1000}$

$=1.6g$

$500ml$ solution contains $20g$

$\therefore$ Total mass of $NaOH$ in $700ml$ solution= $20+1.6=21.6g$

$\therefore$ Molarity= $\frac{21.6/40}{700/1000}=0.77M$