You have a sample of seawater collected off Hawaii, where there is a lot of volcanic activity. As part of an oceanographic study, you are going to analyze it to see if its composition is markedly different from a sample taken farther out in the ocean. The sample of seawater contains, among other solutes, the following concentrations of soluble cations: 0.050 mol · L^–1 Mg^–1 (aq) and 0.010 mol · L^–1 Ca^–1(aq).

(a) Use the information in Table 6I.1 to determine the order in which each ion precipitates as solid NaOH is added and give the molar concentration of OH₂ when precipitation of each begins. Assume that there is no volume change upon addition of the NaOH and that the temperature is 25°C.

(b) If the first compound to precipitate is X(OH)₂, calculate the concentration of X²⁺ ions remaining in solution when the second ion precipitates.

ANTICIPATE In (a), because Ksp for Mg(OH)₂ is so much smaller than K_sp for Ca(OH)₂ and the formulas are analogous, you should expect Mg(OH)₂ to precipitate first. In (b), because much of X²⁺ has precipitated as X(OH)₂ and is no longer in solution, you should expect its concentration to be very small when the second ion precipitates.