Question

The number of moles present in 2 litre of 0.5 M $NaOH$ is:

• A. $2$
• B. $1$
• C. $0.1$
• D. $0.5$

Answer 1

The correct option is B $1$

Molarity is a measure of the concentration of a solute in a solution or of any chemical species, in terms of amount of a substance in a given volume. A commonly used unit for molar concentration used in chemistry is mol/L. A solution of concentration, $1$ mol/L is also denoted as $1$ molar (1 M).

Molar concentration or molarity is most commonly expressed in units of moles of solute per litre of solution. For use in broader applications, it is defined as amount of solute per unit volume of solution, or per unit volume available for the species, represented by lowercase c:

$c=\frac{n}{V}$

where,

$n$= no. of moles of solute.

$V$= volume of solution in liters.

Calculation:

Let the no of moles required be $x$ g.

Molarity $c=0.5M$

The volume of solution in liters is $2$ litres.

Molarity of the given solution is given as $c=\frac{n}{V}$

$c=\frac{x}{2}=0.5⟹x=1$ mole

Number of moles present in $2$ liters of $0.5M$ NaOH is $1$.