Final Concentration After Mixing Solutions JEE Main PYQ

 

❓ Question

20 mL of 2 M NaOH solution is added to 400 mL of 0.5 M NaOH solution. Find the final concentration of the solution.

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✍️ Solution

Moles of NaOH in first solution:

$$n_1=M_1V_1$$
$$n_1=2\times20\times10^{-3}$$
$$n_1=0.04\text{ mol}$$

Moles of NaOH in second solution:

$$n_2=M_2V_2$$
$$n_2=0.5\times400\times10^{-3}$$
$$n_2=0.2\text{ mol}$$

Total moles of NaOH:

$$n_{\text{total}}=0.04+0.2=0.24\text{ mol}$$

Total volume:

$$V_{\text{total}}=20+400=420\text{ mL}$$
$$V_{\text{total}}=0.42\text{ L}$$

Final concentration:

$$M=\frac{\text{Total moles}}{\text{Total volume}}$$
$$M=\frac{0.24}{0.42}$$
$$M=0.5714\text{ M}$$

✅ Final Answer

$$\boxed{0.5714\text{ M}}$$