Question

$1\text{ mole}$ of ${\text{H}}_2$ and $2\text{ moles}$ of ${\text{I}}_2$ are taken initially in a $2\text{ L}$ vessel. The number of moles of ${\text{H}}_2$ at equilibrium is $0.2$ , then the number of moles of ${\text{I}}_2$ and $\text{HI}$ at equilibrium are:
A.$1.2$ , $1.6$
B.$1.8$ , $1.0$
C.$0.4$ , $2.4$
D.$0.8$ , $2.0$

Answer 1

Hint: Here, we have been given that two gases hydrogen and iodine are taken in a fixed volume vessel. The two gases react with each other in a certain ratio to form the product hydrogen iodide. We have to find out the number of moles of iodine and hydrogen iodide, given the number of moles of hydrogen at equilibrium of the reaction.

Complete step by step solution:
The given reaction can be depicted according to the following chemical reaction:
                         ${\text{H}}_2\text{ + }{\text{I}}_2\rightleftharpoons \text{ 2HI}$
Initial concentration before the reaction,
Number of moles of ${\text{H}}_2$ = $1mole$
Number of moles of ${\text{I}}_2$ = $\text{2 }moles$

At equilibrium,
Number of moles of ${\text{H}}_2$ remaining = $0.2$
Number of moles of ${\text{H}}_2$ reacted = $1-0.2=0.8$

Number of moles of ${\text{I}}_2$ reacted = $0.8$
Number of moles of ${\text{I}}_2$ remaining = $2-0.8$
                                                              = $1.2$
$\therefore$ Number of moles of $\text{HI}$ formed = $2\times 0.8$
                                                                = $1.6$

Hence, option (A) is the correct answer.

Additional Information:
“Equilibrium is the state of a reaction at which the concentration of reactants and products do not change with time, i.e. concentrations of reactants and products become constant.”
The important characteristics of equilibrium state can be written as:
1.Equilibrium state can be recognised by the constancy of measurable properties including pressure, concentration, density, colour etc.
2.Equilibrium state can only be achieved in a closed vessel.
3.Equilibrium state is dynamic in nature.
4.Equilibrium state is reversible in nature.
5.At equilibrium state of a reaction,
Rate of forward reaction = Rate of backward reaction.

Note:
Remember the law of mass action which states that the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants at a constant temperature at any given point of time. The molar concentration is also called active mass.