To determine the pH of a 1 × 10⁻⁴ M HCl solution, we start by noting that HCl is a strong acid, which means it completely dissociates in water. Therefore, the concentration of H⁺ ions will also be 1 × 10⁻⁴ M.
The formula for calculating pH is: \[ \text{pH} = -\log[\text{H}^+] \]
Substituting the concentration of H⁺ ions: \[ \text{pH} = -\log(1 \times 10^{-4}) = 4 \]
However, we need to consider that the solution has a significant contribution from the autoionization of water (which is 1 × 10⁻⁷ M). This means that the total concentration of H⁺ ions in the solution is: \[ [\text{H}^+] = 1 \times 10^{-4} , \text{M} + 1 \times 10^{-7} , \text{M} \approx 1 \times 10^{-4} , \text{M} \] (since \(1 \times 10^{-4}\) M is much larger than \(1 \times 10^{-7}\) M).
Thus the pH can be directly calculated as: \[ \text{pH} = 4 \]
Therefore, the correct answer is: 4.
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