Question

What would be the pH of a soda water made by saturating pure water with pure CO₂ at 1.053 atm pressure? You might see that you can neglect certain terms. Hint: solving a cubic equation is involved.

Answer 1

Apply henry laws for solubilty of gases in liquids

H = M/P

or better

M = H*P

where H is the henry constant for a gas = 3.4*10^−2 M/atm

P = partial pressure of CO2

M = concnetration

PArtial pressure of CO2 = 0.00039 (this is a value form databases)

thn

M = (3.4*10^−2)*0.00039 = 0.00001326 mol of CO2 per liter

then

Assume

CO2 + H2O --> H2CO3

and H2CO3 --> 2H+ and CO3-2 in solution

note, the exercise depend on what approach we take

i.e.

assume all CO2 forms acid

we could account for only one H+ and KA1,

we could account fo 2 H+ with KA1 and Ka2

simplest approach is:

pH = -log(0.00001326) = 4.877

pH of cokes are about = 2.52