Question

The water in a shallow lake is at equilibrium with carbon dioxide in the air. The pH was measured to be 7.0. What is the alkalinity of the water in mg/L as CaCO₃? The partial pressure of CO₂ is 1 X 10^-1.5 atm. The Henry's constant for CO₂ is 3.162 X 10^-4 moles/(L-atm) at 25^oC.

Answer 1

As the water in shallow lake is in equlibrium with the CO₂ in air ,it means that water will have some alkalinity due to dissolved carbon dioxide in equlibrium with th water.

So we need to calculate the concentration od CO2 disslved in lake water which is in given by Henry law which states that  the concentration of dissolved gas is proportional to the partial pressure of the gas. For the equilibirium

K = [X] /P

where K is the henry constant and P is the partial pressure of gas

Substituting the values we get

[CO₂]=1*10^-5 mol/L

So alkalinity of the water in mg/L as CaCO₃=(10^-5*60*1000)/0.6= 1mg/L