Question

Solid sodium hydrogen carbonate, NaHCO3, decomposes on heating according to the equation:                     2 NaHCO3(s) <-->...

Solid sodium hydrogen carbonate, NaHCO3, decomposes on heating according to the equation:

                    2 NaHCO3(s) <--> Na2CO3(s) + H2O(g) + CO2(g)

  1. A sample of 1.00 x 102 grams of solid NaHCO3 was placed in a previously evacuated rigid 5.00-liter container and heated to 160 0C. Some of the original solid remained and the total pressure in the container was 7.76 atmospheres when equilibrium was reached. Calculate the number of moles of H2O(g) present at equilibrium.
  2. How many grams of the original solid remain in the container under the conditions described in part (a)?
  3. Write the equilibrium expression for the equilibrium constant, Kp, and calculate its value for the reaction under the conditions in part (a).
  4. If 1.10 x 102 grams of solid NaHCO3 had been placed in the 5.00-liter container and heated to 160 0C, what would the total pressure have been at equilibrium? Explain.
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Answer #1

Ans :-

(a). Given reaction at equilibrium is :

2 NaHCO3(s) <--> Na2CO3(s) + H2O(g) + CO2(g)

Given mass of NaHCO3 = 1.00 x 102 g

Volume = V = 5.00 L

Temperature = T = 160 0C = 433 K

Total pressure of gases mixture = P = 7.76 atm

Number of moles of gases mixture at equilibrium = n = ?

From ideal gas :

PV = nRT

n = PV/RT

n = (7.76 atm).(5.00 L) / (0.0821 L atm K-1mol-1).(433 K)

n = 1.09 mol

Because 2 moles of NaHCO3 gives 1 mol of H2O (g)

So,

Number of moles of H2O at equilibrium = nH2O = 1.09 mol / 2 = 0.545 mol

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(b). Moles of NaHCO3 decomposed = 2 x 0.545 mol = 1.09 mol

So,

Mass of NaHCO3 decomposed = Moles x Gram molar mass of NaHCO3

= 1.09 mol x 84.007 g/mol

= 91.57 g

So,

Mass of Solid remains = 1.00 x 102 g - 91.57 g = 8.43 g

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(c). Expression of Pressure equilibrium constant (Kp) is :

Kp = PH2O.PCO2

Kp = (3.88 atm)(3.88 atm)

Kp = 15.05 atm2

Pressure equilibrium constant (Kp) " is equal to the ratio of partial pressure of products to the partial pressure of reactants raise to power stoichiometric coefficient at equilibrium stage of the reaction ".

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(d). Total pressure after the addition of 1.10 x 102 g of NaHCO3 = 7.76 atm , because addition of solid do not affect the equilibrium stage.

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