PROBLEM 7

1
PROBLEM 7

• If a water is in equilibrium with calcite at 25
  C, what is the calcium concentration in mg L-1
  if the CO32- is 5 mg L-1? Assume activity equals
  molality.
• We need to use the KSP for calcite to solve this
  problem.

2

• We also need to convert mg L-1 CO32- to mol L-1.
• (1 g/1000 mg)(5 mg L-1)/(60.01 g mol-1)
  8.33?10-5
• aCa2 5.13?10-5 mol L-1
• (5.13?10-5 mol L-1)(40.078 g mol-1)(1000 mg/1 g)
  2.05 mg L-1

3
Problem 8

• Calculate the solubility product for the
  dissolution of carbon dioxide in water at 25 C.
  If the dissolved CO2 concentration in a lake at
  the same temperature is 2.2 mg L-1, is the lake
  in equilibrium with atmospheric CO2 (partial
  pressure 10-3.5)? If not, it the gas
  volatilizing from the lake or dissolving into it?

4

• The reaction of interest is as follows
• CO2(g) H2O(l) ? H2CO30
• Here we must realize that CO2(aq) and H2CO30 are
  essentially the same species.
• ?rG? ?fG?(H2CO30) - ?fG?(CO2) - ?fG?(H2O)
• ?rG? -623.2 (-394.4) (-237.1) 8.3 kJ mol-1

5

• Assuming that aH2O ? 1, activity coefficients are
  equal to unity, and for CO2(g), aCO2 ? PCO2, we
  write
• Now we must convert mg L-1 CO2(aq) to mol L-1.
• (2.2 mg L-1 CO2)(1 g/1000 mg)/(44.01 g mol-1)
  5.0?10-5 mol L-1

6

• PCO2 1.41?10-3 atm 10-2.85
• No the lake water is not in equilibrium with
  atmospheric CO2. The partial pressure of CO2 in
  the lake is greater than that in the atmosphere,
  so CO2 will bubble out of the lake.