CHEMISTRY · Single correct · AP · Chemistry · Equilibrium · Equilibrium
Problem
In pure water at 25 °C, the molar solubility of BaSO₄ is 1.0×10−5 M. Students measure the solubility of BaSO₄ in 0.020 M Na₂SO₄ to be 9.0×10−9 M. Using the known Ksp of BaSO₄ (1.1×10−10), which statement best evaluates whether the experimental result agrees with the theoretical prediction?
(a) Because activity coefficients in the 0.020 M Na2SO4 solution are unknown, the theoretical percent decrease cannot be computed, so no valid comparison can be made.
(b) Ignoring the sulfate already present gives a predicted solubility close to that in pure water; the 99.9 % drop observed is therefore far larger than expected and contradicts theory.
(c) The experimental percent decrease (≈99.9 %) is within 5 % of the value predicted from the Ksp, so the results agree with the common-ion effect.
(d) The experimental percent decrease is only about 90 %, well below the theoretical prediction, so the data do not support the common-ion effect.