Question

If you dilute $25.0$ mL of $1.5$ M $HCl$ to $500$ mL, then the molar concentration of each ions becomes $Y\times {10}^{-3}M$. Here $Y$ is:

Answer 1

For dilution, the following equation can be used.
$M_1{V}_1\left(\text{before}\right)=M_2{V}_2\left(\text{after}\right)$
Substituting values in the above expression,
$1.50\times 25.0=M_2\times 500$
$M_2=\frac{1.50\times 25.0}{500}=0.75M=75\times {10}^{-3}$
This is the molar concentration of $HCl$. It is also equal to the molar concentrations of proton and chloride ion.