Question

100 mL of 2 M HCl is diluted to 500 mL. The molarity of the diluted solution is:

• A. 0.2 M
• B. 0.4 M
• C. 0.5 M
• D. 1.0 M

Hint:

Whenever a solution is diluted, the dilution factor is given by $\frac{V_2}{V_1}$.
Here, the volume is increased from $100\text{ mL}$ to $500\text{ mL}$ (a factor of $5$).
Thus, the concentration must decrease by a factor of $5$:
\[ M_2 = \frac{M_1}{5} = \frac{2}{5} = 0.4\text{ M} \]
This mental math technique helps save time in the exam.

Answer 1

The Correct Option is B

Step 1: Dilution formula.
$M_1V_1 = M_2V_2 \Rightarrow 2 \times 100 = M_2 \times 500$
\[ \boxed{M_2 = 0.4 \text{ M}} \]