CE 680 20 November 1991

# SECOND EXAM

Closed book, two pages of notes allowed.

Answer all questions. Please state any additional assumptions you made, and show all work.

1. (50%) Carbonate System. A groundwater that is at pH 7.4 and is in equilibrium with a partial pressure of CO2 of 10-2.0 atmospheres is pumped to the surface. What is its pH once it has reached equilibrium with the bulk atmosphere?

2. (50%) Precipitation and Dissolution: Manganese System

A. (35%) Prepare a pC-pH diagram for manganese hydrolysis species in equilibrium with Mn(OH)2 precipitate. You may limit your figure to the pH region from 10 to 14. Include a line approximating the total soluble manganese concentration.

B. (15%) If 100 g of MnS(s) and 0.01 mole of NaS(s) is added to 1000 mL of water buffered at pH 11.9, how much of the MnS will dissolve? Will Mn(OH)2 precipitate? Assume the system remains at pH 11.9, and that it is at equilibrium and closed to the atmosphere.

Important Equilibrium constants:

Log K

Mn+2 + H2O = MnOH+ + H+

-10.6

Mn+2 + 3H2O = Mn(OH)3 + 3H+

-35

Mn+2 + 4H2O = Mn(OH)42 + 4H+

-48.3

Mn(OH)2(s) + 2H = Mn+2 + 2H2O

15.2

MnS(s) = Mn+2 + S-2

-13.5

H2S(aq) = H+ + HS-

7.02

HS- = H+ + S-2

13.9