Question

Atmospheric carbon dioxide concentrations may reach 500 parts per million within the next 30 years. Calculate the pH of rainwater in equilibrium with carbon dioxide at 500 ppm. Show all relevant equations and calculations.

Answer 1

500 ppm of CO2 -----> 500 grams in 10⁶ ml of air

Moles = 500/44 = 11.36 moles

Molarity = Moles/volume in litres = 1000x (11.36/10⁶ ) = 1.136 x10^-3 in air

The solubility of CO2 in in 1 litre of water at 25oc = 1.5 gram per litre of water

SO Molairty of CO2 in rain water = (1.5/44)/1 = 0.034

pka of H2CO3 = 6.7

so pH = 1/2pka - 1/2 log(c) = 1/2(6.7) - 1/2log(0.034)

                                         = 4.08