Question

The volume of water that must be added to a mixture of 250 ml of 0.6 M $HCl$ and 750 ml of 0.2 M $HCl$ to obtain 0.25 M solution of $HCl$ is

• A. 750 ml
• B. 100 ml
• C. 200 ml
• D. 300 ml

Answer 1

The correct option is C

200 ml

We can see that the number of moles of
$HCl$ should remain constant, as per the law of conservation of mass.

Since we have molarity, we can calculate number of moles using,

No. of moles = Molarity $\times$ volume of solution

Initial Number of moles = 250 $\times$ 0.6 + 0.2 $\times$ 750

Final number of moles = 0.25 (250 + 750 + $V_{required}$) Equating, we get

$V_{required}=\frac{0.6\times 250+0.2\times 750}{0.25}-\left[1000\right]=200ml$