Relative Concentrations

Reactant A undergoesdissociation into products B and C according to the following equilibriumreaction equation
A(g) ⇌B(g) + C(g)
A rigid vessel, initially containing 0.072 mol/L of pure A, is allowed toequilibrate at 31°C.
What would be the value of the equilibrium constant (Keq expressed in terms ofthe molar concentrations) if the equilibrium concentrations of all species areequal?

Solution :
A(g) ⇌B(g)+ C(g)

At eqm :
Let [B] = [C] = y mol/L
Then, [A] = (0.072 - y) mol/L

[A] = [B]
0.072 - y = y
y = 0.036 Keq = [B][C]/[A] = y = 0.036


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What would be the value of the equilibrium constant (Keq expressed in terms ofthe molar concentrations) if, at equilibrium, the concentration of A is 1.0%that of B? (No units required.)

Solution :
A(g) ⇌ B(g)+ C(g)

At eqm :
[A] + [B] = 0.072 mol
1.0%[B]+ [B] = 0.072 mol
[B] = 0.072/1.01 mol

[C] = [B] = 0.072/1.01 mol

[A] = (0.072/1.01) x 1% = 0.00072/1.01 mol

Keq = [B][C]/[A] = (0.072/1.01)²/(0.00072/1.01) = 7.13