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When the pH is lowered from 7 to 2, the solubility of a slightly soluble salt (MX) derived from a weak acid (HX) rises from 10⁻⁴ mol L⁻¹ to 10⁻³ mol L⁻¹. What is the pKa value of HX?

1. 3
2. 4
3. 5
4. 2

Correct answer: 4

Solution

The increase in solubility of MX in acidic conditions is due to the common ion effect. The pKa of HX is determined using the relationship between solubility and pH, yielding a value of 4.

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