What is the pH of a 10^(-7) M solution of HCl?

Correct option is B

Given:

• Concentration of HCl = 10^-7M
• Auto-ionization of water must be considered since the concentration of HCl is very low.

Step 1: Consider the Contribution of Water

Water itself ionizes to give:

H2O⇌H⁺+OH^$$-$$

The concentration of H+H^+H+ from water is:

[H⁺]water =10^-7M

Since HCl is a strong acid, it completely dissociates:

HCl→ H⁺ + Cl^-

So, the total hydrogen ion concentration is:

[H⁺]total = [H⁺]water + [H⁺] HCl

= 10^-7 + 10^-7 ​

= $$2 \times 10^{-7} \, M$$

Step 2: Calculate pH

pH=−log⁡[H⁺]

pH=−log(2×10^-7​)

Using logarithm properties:

log(2×10^-7)=log2+log10^$$-7$$

Approximating $\log 2\approx 0.301$:

$$\text{pH}=-(0.301-7)$$$$\text{pH}$$

=6.70≈6.79

Correct Answer:

(2) 6.79

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