Carbon dioxide dissolves in water to form carbonic acid, which is primarily diss

Question

Carbon dioxide dissolves in water to form carbonic acid, which is primarily dissolved CO2. Dissolved CO2 satisfies the equilibrium equation

CO2(g) [double arrow] CO2(aq) K=0.032 M. atm ^-1

The acid dissociation listed in most standard reference texts for carbonic acid actually apply to dissolved CO2. For a CO2 partial pressure of 7.0*10^-4 atm in the atmosphere, what is the ph of water with the atmosphere?

I really need help with this question. I'm not sure about how to approach this problem.

Thank you.

Explanation / Answer

Since carbonic acid is primarily dissolved CO2, the concentration of H2CO3 can be taken as equal to that of dissolved CO2,
[H2CO3]=[CO2aq]
The equilibrium equation for the dissolution of gaseous CO2 can be used to calculate the [CO2(aq)] and, hence, the [H2CO3]. The [H2CO3] will then be used to calculate the pH. Dissolved CO2 satisfies the equilibrium equation
pH=5.609

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