10. An 8000 m 3 lake has a pH of 4. For this problem, assume no water is added o

Question

10. 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 For this problem, assume no water is added or withdrawn from the lake.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+.

Explanation / Answer

A

pH = 4

So [H+] = 10^-4 M

B.

INITIAL CONCENTRATION OF OH- = 10^-10 M

at pH = 6

[H+] = 10^-6 M

FINAL CONCENTRATION OF OH- = 10^-8 M

SO CONCENTRATION OF OH- REQUIRED = 10^-8 - 10^-10 = 9,9 * 10^-9 M

C.

CaCO3(s) + 2 HCl(aq) = H2O(l) + CO2(g) + CaCl2(aq)

D.

conc of lake = 10^-4 M H+

moles of H+ = 10^-4 * 8*10^6 = 800 moles

no of moles of caco3 = 1/2 * moles of H+  (ACCORDING TO ABOVE CHEMICAL REACTION)

= 400 mol

SO mass of caco3 = 400 * 100.08 = 40034.76 g = 40.034 kg

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