Question

There are two samples of $HCl$ having concentrations of 1 M and 0.25 M. Find amongst the given options, the combination of these samples needed to prepare a 0.75 M $HCl$ solution assuming no extra water is used.

• A. 20 mL, 10 mL
• B. 100 mL, 60 mL
• C. 50 mL, 20 mL
• D. 50 mL, 35 mL

Answer 1

The correct option is A 20 mL, 10 mL

Let the volume of 1 M solution be $V_1$ and volume of 0.25 M solution be $V_2$.
Conserving moles before and after dilution:
$M_1{V}_1+M_2{V}_2=MV$
where $M$ and $V$ are the final molarity and volume.
$0.75\times (V_1+V_2)=(1\times V_1)+(0.25\times V_2)$
On solving, $\frac{V_1}{V_2}=\frac{2}{1}$
From the options, option (a) will match the ratio of volumes.
Therefore, 20 mL of 1 M solution and 10 mL of 0.25 M solution should be mixed.