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

The correct option is A. 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

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