Question

Sodium carbonate, Na₂CO₃(s), can be prepared by heating sodium bicarbonate, NaHCO₃(s).

2NaHCO₃(s) ⇌ Na₂CO₃(s) + CO₂(g) + H₂O(g), K_p = 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?

Answer 1

Let's write the overall reaction again:

2NaHCO_3(s) <----------> Na₂CO_3(s) + CO_2(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