CAIE A-Level Chemistry 25.1.8 Ksp Expressions and Units

Practise deriving Ksp expressions and units from balanced dissolution equations for ionic solids.

Exam points

dissociate the solid into correctly charged aqueous ions with balanced coefficients
use those coefficients as concentration powers, such as [M²+][X−]² for MX2
calculate the mol dm−3 unit power from the sum of exponents rather than memorising one unit

[Maximum number: 2]

Manganese(II) hydroxide, $\mathrm{Mn}(\mathrm{OH})_{2}$ , is only slightly soluble in water.

The solubility of $\mathrm{Mn}(\mathrm{OH})_{2}$ in water is $3.28 \times 10^{-3} \mathrm{gdm}^{-3}$ at 298 K .

Write an expression for the $K_{\mathrm{sp}}$ of $\mathrm{Mn}(\mathrm{OH})_{2}$ . Give the units of $K_{\mathrm{sp}}$ .

K_{s p}=

units =

$\text { M1: } K_{\mathrm{sp}}=\left[\mathrm{Mn}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}$

M2: units $=\mathrm{mol}^{3} \mathrm{dm}^{-9}$

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25.1.8—Write an expression for Ksp question 1