U8.1 · Introduction to Acids and BasesPractice · 酸碱入门
Lessons covered:U8.1 Introduction to Acids and Bases
- 1Numeric
At 25 °C a solution has [H₃O⁺] = 4.0 × 10−9 M. What is its pH?
ExplanationpH = −log[H₃O⁺] = −log(4.0 × 10−9) = 8.40. Two significant figures in the concentration give two decimal places in pH. pH > 7, so the solution is basic.
- 2Numeric
At 25 °C a solution has pH = 3.25. What is [OH⁻]?
× 10MExplanationpOH = 14.00 − 3.25 = 10.75, so [OH⁻] = 10−10.75 = 1.8 × 10−11 M. An acidic solution has very little OH⁻, but not none.
- 3Multiple choice
How many times greater is [H₃O⁺] in a solution at pH 3.0 than in one at pH 6.0?
ExplanationEach drop of 1 pH unit is a tenfold increase in [H₃O⁺]. Three units apart means 10³ = 1000 times. pH is a log scale, so the values can’t simply be divided.
- 4Multiple choice
At 0 °C the pKw of water is 14.95, and pure water has a pH of about 7.5. Is pure water at 0 °C acidic, basic, or neutral?
ExplanationNeutral means [H₃O⁺] = [OH⁻]. Pure water always makes equal amounts of H₃O⁺ and OH⁻, so it is neutral at any temperature. At 0 °C Kw is smaller, so neutral pH = 14.95 ÷ 2 ≈ 7.5; “pH 7 is neutral” holds only at 25 °C.
- 5Numeric
At 25 °C, a solution has [H₃O⁺] equal to twice [OH⁻]. What is its pH?
Explanation[H₃O⁺][OH⁻] = 1.0 × 10−14 and [OH⁻] = [H₃O⁺]/2, so [H₃O⁺]² = 2.0 × 10−14, [H₃O⁺] = 1.41 × 10−7 M, and pH = 6.85. Just under 7: slightly more H₃O⁺ than OH⁻, so the solution is very slightly acidic.
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