Question

Acid rain over the Great Lakes has a pH of about 4.5 while acid rain over the West Coast has a pH of about 5.4. The lakes are then acidified by rainfall but are after treated through a neutralizing process called liming. This process involves adding limestone to the lake in large quantities.

a. What are the [H3O+] concentrations in each lake?
b. How many more times concentrated is the acid rain over the Great Lakes?
c. How much limestone in kg would need to be added to completely neutralize a lake on the Great Lakes that is 4.3 billion liters? (Hint: you need to look at the reaction between the lime and the H+ to get the right stoichiometric ratio)

Answer 1

a) Great lakes= 3.16×10^-5 M

West coast=3.98×10^-6 M

b) The Great lakes is 7.9 times more concentrated than the West Coast

c)13588Kg of limestone

Step-by-step explanation:

The oxonium ion concentration in the Great lakes is obtained from:

pH= -log [H3O^+]

Hence [H3O^+] = Antilog(- pH)

Since pH= 4.5

[H3O^+] = Antilog(- 4.5)

[H3O^+] = 3.16×10^-5 M

For the West Coast

pH= -log [H3O^+]

Hence [H3O^+] = Antilog(- pH)

Since pH= 5.4

[H3O^+] = Antilog(- 5.4)

[H3O^+] = 3.98×10^-6 M

b) To compute how many times more concentrated the Great lakes is over the west coast,

3.16×10^-5/3.98×10^-6 = 7.9

The Great lakes is 7.9 times more concentrated than the West Coast

c) number of moles of hydrogen ions required for the reaction=

3.16×10^-5 M × 4.3×10^9 L = 13.588×10^4 moles

H3O^+ = H^+

From the reaction:

CaCO3(s) + H3O^+(aq) ------> Ca^2+(aq) + HCO3^-(aq) + H2O(l)

1 mole of limestone reacts with 1 mole of hydrogen (oxonium) ions thus

x moles of limestone will react with 13.588×10^4 moles

x= 13.588×10^4 moles of limestone.

Molar mass of limestone = 100gmol-1

Hence mass of limestone= 13.588×10^4 moles × 100gmol-1 = 13588000g or 13588Kg