Q. A 0.1 M solution of a weak acid (Ka = 1.0 × 10⁻⁵) has a pH of:

एक कमजोर एसिड (Ka = 1.0 × 10⁻⁵) के 0.1 M घोल का pH होता है:

A
3.0
B
3.5
C
5.0
D
7.0

Explanation

Use the formula for weak acid:

[H+]=Ka×C=1.0×10−5×0.1=1.0×10−6=1.0×10−3[H^+] = \sqrt{Ka × C} = \sqrt{1.0 × 10^{-5} × 0.1} = \sqrt{1.0 × 10^{-6}} = 1.0 × 10^{-3} pH=−log⁡(1.0×10−3)=3.0\text{pH} = -\log(1.0 × 10^{-3}) = 3.0

But more accurately:

[H+]≈Ka×C=1.0×10−5×0.1=3.16×10−4⇒pH≈−log⁡(3.16×10−4)≈3.5[H^+] ≈ \sqrt{Ka × C} = \sqrt{1.0 × 10^{-5} × 0.1} = 3.16 × 10^{-4} \Rightarrow \text{pH} ≈ -\log(3.16 × 10^{-4}) ≈ 3.5

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