Barrons SAT Subject Test Chemistry, 13th Edition

64. (B) Ksp = [Ba2 +][SO 4 2−]

If [Ba2+] = 4.0 × 10−5, then [SO 4 2−] must also equal the same amount, so

Ksp = [4.0 × 10−5][4.0 × 10−5] = 16 × 10−10 or 1.6 × 10−9.