Question

Equal volumes of two solutions A and B of a strong acid having pH = 6.0 and pH = 4.0 respectively are mixed together. The pH of the new solution will be in the range

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

Hint:

Mixing acids: \begin{itemize} \item Convert pH → concentration first. \end{itemize}

Answer 1

The Correct Option is C

Concept: Strong acid ⇒ full ionization. Use \( [H^+] = 10^{-pH} \). Step 1: {\color{red}Concentrations.} \[ [H^+]_A = 10^{-6}, \quad [H^+]_B = 10^{-4} \] Step 2: {\color{red}Equal volume mixing.} Average concentration: \[ [H^+] = \frac{10^{-6} + 10^{-4}}{2} \approx \frac{10^{-4}}{2} = 5\times10^{-5} \] Step 3: {\color{red}Find pH.} \[ \text{pH} = -\log(5\times10^{-5}) = 5 - \log 5 \approx 4.3 \] So between 4 and 5.