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Question

Identify the equilibrium reaction in which KC has no units.

A
Decomposition of NH3
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B
Decomposition of HI
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C
Decomposition of PCl5
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D
All of the above
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Solution

The correct option is B Decomposition of HI
The units of Equilibrium constant KC, will depend on the number of moles of reactants and products. When the total number of moles of products is equal to the total number of moles of reactants, then KC has no units.

In the question the reactions are expressed as:

Decomposition of HI
2HI(g) H2(g)+I2(g)

Decomposition of PCl5
PCl5(g) PCl3(g)+Cl2(g)

Decomposition of NH3
2NH3(g) N2(g)+3H2(g)

In the above reactions, only in the reaction of decomposition of HI, moles of reactants and product are the same.

Hence Equilibrium constant KC has no units.


Theory:

Units of Equilibrium constant Kc:

The equilibrium constant for a general reaction,
a A+b Bc C+d D

Kc=[C]ceq[D]deq[A]aeq[B]beq

where [C]ceq[D]deq[A]aeq and [B]beq are the equilibrium concentrations of the reactants and products.

Kc=(mol/L)c(mol/L)d(mol/L)a(mol/L)b

Unit of Kc(mol/L)(c+d)(a+b)

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