The acid dissociation constants listed in most standard reference texts for carb

Question

The acid dissociation constants listed in most standard reference texts for carbonic acid actually apply to dissolved CO2.

Carbon dioxide dissolves in water to form carbonic acid, which is primarily dissolved CO2. Dissolved CO2 satisfies the equilibrium equation CO2(g) CO2(aq) K=0.032 The acid dissociation constants listed in most standard reference texts for carbonic acid actually apply to dissolved CO2. For a CO2 partial pressure of 4.1x104 bar in the atmosphere, what is the pH of water in equilibrium with the atmosphere? (For carbonic acid Kal-4.46x 10-7 and Ka2 = 4.69 x 10-11). Number

Explanation / Answer

We can get this via

P-CO2 = 4.1*10^-4 bar = ( 4.1*10^-4)*0.986923 = 0.00040463 atm

KCO2 = henry constant

[CO2(aq)] = KCO2*PCO2 = (3.4*10^-2)(0.00040463 ) =  0.00001375 = 1.375*10^-5 M

now..

Ka= [H+]^2 / [CO2(aq9]

4.46*10^-7 = [H+]^2 /(( 1.375*10^-5 ))

[H+] = sqrt( (1.375*10^-5 )*(4.46*10^-7 ))

[h+] = 2.47*10^-6

pH = -log( 2.47*10^-6) = 5.61

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