In which ratio of volume 0.4 M HCl and 0.9 M HCl are to be mixed such that the concentration of the resultant solution becomes 0.7 M ?

The correct option is B 2:3
Given: Molarity of the first solution=0.4 M

Molarity of the second solution = 0.9 M

Solution: Let the volume of 0.4 M HCl solution be $V_1$ and volume of solution 0.9 M HCl be $V_2$.

According to the question,

$0.4V_1+0.9V_2=0.7(V_1+V_2)$ (1)

On simplifying we get,

$0.4V_1-0.7V_1=0.7V_2-0.9V_2$

$-0.3V_1=-0.2V_2$

$\frac{V_1}{V_2}=\frac{2}{3}$

Hence, the correct option is (B).