Arrange the following mixtures in increasing order of their resultant molarity

(A) 20 mL of 0.5 M HCl + 30 mL of 0.3 M HCl
(B) 30 mL of 0.3 M HCl + 30 mL of 0.3 M HCl
(C) 20 mL of 0.5 M HCl + 20 mL of 0.3 M HCl
(D) 20 mL of 0.3 M HCl + 30 mL of 0.5 M HCl

Choose the correct answer from the options given below:

(B), (A), (C), (D)

The correct answer is Option (3) → (B), (A), (C), (D)

To find the resultant molarity ($M_{\text{res}}$​) of mixtures:

$M_{\text{res}} = \frac{M_1V_1 + M_2V_2}{V_1 + V_2}$

Let's calculate for each mixture:

(A) 20 mL of 0.5 M + 30 mL of 0.3 M

$M_{\text{res}} = \frac{0.5 \times 20 + 0.3 \times 30}{20+30} = \frac{10 + 9}{50} = \frac{19}{50} = 0.38\text{ M}$

(B) 30 mL of 0.3 M + 30 mL of 0.3 M

$M_{\text{res}} = \frac{0.3 \times 30 + 0.3 \times 30}{60} = \frac{18}{60} = 0.3\text{ M}$

(C) 20 mL of 0.5 M + 20 mL of 0.3 M

$M_{\text{res}} = \frac{0.5 \times 20 + 0.3 \times 20}{40} = \frac{10 + 6}{40} = \frac{16}{40} = 0.4\text{ M}$

(D) 20 mL of 0.3 M + 30 mL of 0.5 M

$M_{\text{res}} = \frac{0.3 \times 20 + 0.5 \times 30}{50} = \frac{6 + 15}{50} = \frac{21}{50} = 0.42\text{ M}$

Increasing order:

$B (0.3) < A (0.38) < C (0.4) < D (0.42)$