Mono Lake in eastern California is a rather unusual salt lake. The lake has no outlets; water leaves only by evaporation. The rate of evaporation is great enough that the lake level would be lowered by three meters per year if not for fresh water entering through underwater springs and streams originating in the nearby Sierra Nevada mountains. The principal salts in the lake are the chlorides, bicarbonates, and sulfates of sodium. An approximate “recipe” for simulating the lake water is to dissolve 18 tablespoons of sodium bicarbonate, 10 tablespoons of sodium chloride, and 8 teaspoons of Epsom salt (magnesium sulfate heptahydrate) in 4.5 liters of water (although the lake water actually contains only trace amounts of magnesium ion). Assume that 1 tablespoon of any of the salts weighs about 10 g. (1 tablespoon = 3 teaspoons.)

(a) Expressed as grams of salt per liter, what is the approximate salinity of Mono Lake? How does this salinity compare with seawater, which is approximately 0.438 M NaCl and 0.0512 M MgCl₂?

(b) Estimate an approximate pH for Mono Lake water. How does your estimate compare with the observed pH of about 9.8? Actually, the recipe for the lake water also calls for a pinch of borax. How would its presence affect the pH? [Borax is a sodium salt, Na₂B₄O₇ · 10 H₂O, related to the weak monoprotic boric acid (pK_a = 9.25).

(c) Mono Lake has some unusual limestone formations called tufa. They form at the site of underwater springs and grow only underwater, although some project above water, having formed at a time when the lake level was higher. Explain how the tufa form. What chemical reaction(s) is(are) involved?