Calculate the pH of buffer solution containing 0.05 mol NaF per litre and 0.015 mol HF per litre. [$K_a = 7.2 imes 10^{-4}$ for HF]

Question

Calculate the pH of buffer solution containing 0.05 mol NaF per litre and 0.015 mol HF per litre. [$K_a = 7.2 	imes 10^{-4}$ for HF]

Accepted Answer

> [!TLDR]
> p$K_a$(HF) $= 3.1427$ and $\log(0.05/0.015) = 0.5224$ give pH $= 3.6651 \approx 3.67$.

**Step 1.** NaF + HF is an acidic buffer (weak acid HF with its salt NaF), so the Henderson-Hasselbalch equation (Eq. 3.24) applies: $	ext{pH} = 	ext{p}K_a + \log_{10}\dfrac{[	ext{salt}]}{[	ext{acid}]}$.

**Step 2.** $	ext{p}K_a = -\log_{10}(7.2	imes10^{-4}) = 4 - \log_{10}7.2 = 4 - 0.8573 = 3.1427$.

**Step 3.** $\log_{10}\dfrac{[	ext{salt}]}{[	ext{acid}]} = \log_{10}\dfrac{0.05}{0.015} = \log_{10}3.33 = 0.5224$.

**Step 4.** $	ext{pH} = 3.1427 + 0.5224 = 3.6651 \approx 3.67$.

> [!ANSWER]
> pH of the buffer $= 3.6651 \approx 3.67$ -- digit-for-digit the textbook's printed final.

Q.Calculate the pH of buffer solution containing 0.05 mol NaF per litre and 0.015 mol HF per litre. [ $K_a = 7.2 \times 10^{-4}$ for HF]

Concept understanding — Buffer Solutions

Q.Calculate the pH of buffer solution containing 0.05 mol NaF per litre and 0.015 mol HF per litre. [Ka=7.2×10−4K_a = 7.2 \times 10^{-4}Ka​=7.2×10−4 for HF]

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Q.Calculate the pH of buffer solution containing 0.05 mol NaF per litre and 0.015 mol HF per litre. [ $K_a = 7.2 \times 10^{-4}$ for HF]

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