Sodium carbonate, Na2CO3(s), can be prepared by heating sodium bicarbonate, NaHCO3(s).
2NaHCO3(s) ⇌ Na2CO3(s) + CO2(g) + H2O(g), Kp = 0.264 at 179 °C
If a sample of NaHCO3 is placed in an evacuated flask and allowed to achieve equilibrium at 179 °C, what will the total gas pressure (in atm) be?
Let's write the overall reaction again:
2NaHCO3(s) <----------> Na2CO3(s) + CO2(g) + H2O(g)
Now, in equilibrium the only compounds to have concentrations and/or pressure will be those compounds that are in a gaseous state or aqueous state. Solids and liquids are not taking account in the equilibrium expression because it's concentration is constant and near to a value of 1, so it will not contribute for the equilibrium.
With this, the expression of equilibrium for this reaction is:
Kp = pCO2 * pH2O
The total pressure will be:
pT = pCO2 + pH2O
But pCO2 = pH2O
So pT = 2pCO2
0.264 = (pCO2)2
pCO2 = pH2O = (0.264)1/2 = 0.514 atm
pT = 2*0.514
pT = 1.028 atm
Hope this helps
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