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Question:

The equilibrium constant at 298 K for a reaction A + B ⇌ C + D is 100. If the initial concentration of all the four species were 1 M each, then equilibrium concentration of D (in mol L⁻¹) will be:

0.182

1.818

0.818

1.182

Solution:

A + B → C + D
1 1 1 1 : initially
1-x 1-x 1+x 1+x: at equilibrium
Kc = (1+x)²/(1-x)²
100 = (1+x)²/(1-x)²
1+x/1-x = 10
1+x = 10(1-x)
1+x = 10 - 10x
11x = 9
x = 9/11
Moles of D = 1 + 9/11 = 20/11 = 1.818