Q10 / 155 · WBJEE 2025 · Chemistry

Equal volume of two solutions A and B of a strong acid having $\mathrm{pH}=6.0$ and $\mathrm{pH}=4.0$ respectively are mixed together to form a new solution. The pH of the new solution will be in the range

  • A
    between 5 and 6
  • B
    between 6 and 7
  • Correct answer
    C
    between 4 and 5
  • D
    between 3 and 4

Solution

$\mathrm{pH}=6 \quad \therefore\left[\mathrm{H}^{+}\right]=10^{-6} \mathrm{M}$ Let volume is 1 L $1 \mathrm{~L} 10^{-6} \mathrm{M}$ SA contains $10^{-6}$ moles $\mathrm{H}^{+}$ $\mathrm{pH}=4 \quad \therefore\left[\mathrm{H}^{+}\right]=10^{-4} \mathrm{M}$ $1 \mathrm{~L} 10^{-4} \mathrm{M}$ SA contains $10^{-4}$ moles $\mathrm{H}^{+}$ $\therefore$ Total $\mathrm{H}^{+}=10^{-6}+10^{-4}=1.01 \times 10^{-4}$ moles $\begin{aligned} & \therefore\left[\mathrm{H}^{+}\right]=\frac{1.01 \times 10^{-4}}{2}(\mathrm{M})=5.05 \times 10^{-5} \\ & \mathrm{pH}=-\log \left(5.05 \times 10^{-5}\right)=4.29 \end{aligned}$

Hence, the correct answer is option: 'C'.

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