Question

200 mL of a 4 % (w/v) $NaOH$ solution is diluted up to 500 mL. Find the resultant molarity of the solution.

• A. 0.4 M
• B. 0.5 M
• C. 0.8 M
• D. 0.3 M

Answer 1

The correct option is A 0.4 M

4% (w/v) solution means 4 g of $NaOH$ is present in 100 mL of the solution.
$\text{Number of moles of}NaOH=\frac{\text{given mass of}NaOH}{\text{molar mass}}$
$=\frac{4}{40}=0.1$

$\text{Molarity}=\frac{\text{number of moles of solute}}{\text{volume of solution in mL}}\times 1000$

$\text{Molarity}=\frac{0.1}{100}\times 1000=1M$
Conserving moles before and after dilution:
$M_1{V}_1=M_2{V}_2$
Where, $M_1=1M$ and $V_1=200mL$
$M_2$ and $V_2$ are the molarity and volume of the resultant solution.
$(1\times 200)=(M\times 500)$
$M=0.4M$

Molarity of the resultant solution is $0.4M$.