1 M HCl and 2 M HCl are mixed in volume ratio of 4:1. The resulting HCl solution is diluted to double its original volume. What is the final molarity of HCl solution?

The correct option is C 0.6 M
Let volume of 1 M HCl solution is 4V Litres and 2 M HCl solution is V Litres. Then, molarity after mixing can be given as,
$M^{{}^{\prime }}=\frac{M_1\times V_1+M_2\times V_2}{V^{{}^{\prime }}}$
where, $M^{{}^{\prime }}$ and $V^{{}^{\prime }}$ are molarity and volume of solution mixture,
$V_1$ and $V_2$ are 4V and V respectively.
So, $M^{{}^{\prime }}=\frac{1\times 4V+2\times V}{4V+V}=1.2M$

Now, final volume is doubled after dilution, So final volume$\left(V_f\right)=2V^{{}^{\prime }}=10V$
$\therefore M_f=\frac{M^{{}^{\prime }}\times V^{{}^{\prime }}}{V_f}=0.6M$