Question

In a reaction vessel of 2 liter capacity 3 moles of $N_2$ reacts with 2 moles of $O_2$ to produce 1 mole of NO. What is the molar concentration of $N_2$ at equilibirium ?

• A. 1.25
• B. 1.5
• C. 0.75
• D. 2

Answer 1

Answer: (A)

1.25

The equilibrium reaction is $N_2+O_2\iff 2NO$.
The following table list the number of moles of various species.

	$N_2$	$O_2$	$NO$
Initial	3	2	0
Change	-x	-x	2x
Equilibrium	3-x	2-x	2x

But, at equilibrium, the number of moles of NO are $2x$ or $1$. Thus $x=0.5$.
The number of moles of nitrogen at equilibrium are $3-x=3-0.5=2.5$.
The total volume is 2L.
The equilibrium concentration of nitrogen is $\left[N_2\right]=\frac{2.5}{2}=1.25$.