A swimming pool was sufficiently alkaline so that the carbon dioxide absorbed from the air produced a solution in the pool that was 2 × 10-4 M in carbonate ion. If the pool originally contained 4 × 10-3 M Mg2+, 6 × 10-4 M Ca2+, and 8 × 10-7 M Fe2+, then the precipitate that was formed consisted of: [Ksp values are: CaCO3, 4.7 × 10-9; MgCO3, 4.0 × 10-5; FeCO3, 2.0 × 10-11]