Question

An 8000 m³ lake has a pH of 4. For this problem, assume no water is added or withdrawn from the lake.

                a. What is the molarity of H⁺ in the lake?

                b. How many moles of OH^- would be needed to raise the pH of the lake to 6?

                c. One method for neutralizing acids in surface water is to add lime (CaCO₃). Write a balanced equation for the

                neutralization of H⁺ with limestone. (Hint: see problem 7)

d. If this OH^- all comes from limestone, how many kg of limestone will be needed? Assume that all of the

limestone reacts with H⁺.

Answer 1

a)

M = mol/ Volume (in liters)

if pH = 4

the

[H+] = 10^-pH = 10^-4 mol of H+ per liter

b)

moles of OH- needed to raise pH = 6

[H+] = 10^-4 to 10^-6

[H+] = [H+final] - [H+initial] = 10^-6 - 10^-4 = 0.000099 M

so

0.000099 mol of OH- per liter are required

V = 8000 m3 = 8000*10^3 L

so

M = mol/V

mol = MV = (0.000099)(8000*10^3) = 792 mol of OH- requried

c)

equation

CaCO3(s) + 2 H+ (aq) = H2O(l) + CO2(g) + Ca+2(aq)

d)

find limestone required

1 mol of CaCO3 = 2 mol of OH-

792 mol of OH- was required so

792 /2 = 396 mol of CaCO3 required now

mass = mol*MW = 396*100 = 39600 grams = 39.6 kg of CaCO3 required