Question

To 20 mL, 1.25 M HCl solution 30 mL of water is added to this resultant solution and again 75 mL of 0.25 M HCl in added. The molarity of the resultant solution is

• A. 0.40 M
• B. 2.10 M
• C. 0.35 M
• D. 5.5 M

Answer 1

The correct option is C 0.35 M

In first case
$M_1{V}_1=M_2{V}_2$
here, $M_1=1.25M;V_1=20mL;V_2=50mL$
So $M_2=\frac{1.25\times 20}{50}=0.5M$
Now, to this 0.5 M of 50 mL HCl solution, 75 mL of 0.25 M HCl is added.
Using,
$M_3=\frac{M_1{V}_1+M_2{V}_2}{V_3}$ where $V_3=50+75=125$ mL
$M_3=\frac{(0.5\times 50)+(75\times 0.25)}{125}{M}_3=0.35M$