WBJEE 2025 Physics and Chemistry Papers

Since both solutions are strong acids, we assume complete dissociation. The hydrogen ion concentration in each solution is given by:
[H+]=10−pH.
For Solution A pH=6.0):
[H+]A=10−6M
For Solution B (pH=4.0):
[H+]B=10−4M.
When equal volumes of the two solutions are mixed, the total hydrogen ion concentration in the new solution is the average of the two:[H+]‌mix ‌=‌

[H+]A+[H+]B

2

=‌

10−6+10−4

2

.
Calculate the numerator:
10−6+10−4=0.000001+0.0001≈0.000101.
Then divide by 2 :
[H+]mix≈‌

0.000101

2

≈5.05×10−5M.
[H+]‌mix ‌≈‌

0.000101

2

≈5.05×10−5M.
Now, we convert this concentration back to pH :
pHmix=−log10(5.05×10−5).
Breaking the logarithm into parts:
Since log10(5.05)≈0.7, we get:
pH‌mix ‌≈5−0.7=4.3.
Thus, the pH of the new solution is approximately 4.3 , which lies between 4 and 5 .
Therefore, the correct answer is:
Option C: between 4 and 5.