Question

How many moles and how many grams of NaCl are present in 250 mL of a 0.5 M NaCl solution?
(A) 0.125 mol; 7.32 g
(B) 7.32 mol; 0.125 g
(C) 0.125 mol; 0.125 g
(D) 7.32 mol; 7.32 g

Answer 1

Hint: Understand the type of concentration and molarity. The volume of the solution, as well as molarity, is mentioned in the question. We can find the number of moles and then eventually mass of NaCl added by using the below formula.
$\text{M = }{\displaystyle \frac{\text{n}}{\text{V}}}$
Where,
$M$ is the molarity of the solution
$n$ is the number of moles of solute
$V$ is the volume of the solution in litres

Complete step by step solution:
Molar concentration also is known as molarity is a measure of the concentration of a chemical species or substance, in particular of a solute in a solution. It is measured in terms of the amount of substance per unit volume of solution.
In chemistry, the most commonly used unit for molarity is moles per litre, having the unit symbol mol/L. This is also the SI unit representing molarity.
We have been provided with the values of molarity and volume of solution.
$M=$ $0.5M$
$V=$ $0.250L$
Substituting these values in the equation mentioned in hint, we get,
$\text{M = }{\displaystyle \frac{\text{n}}{\text{V}}}$
$\text{0}\text{.5 = }{\displaystyle \frac{\text{n}}{0.250}}$
$n=$ $0.125$ $mole$.
We know that 1 mole of any substance contains the amount of mass equal to its molar mass. So, 1 mole of NaCl has a mass of 58.5 g.
Similarly, 0.125 mole of NaCl will have a mass,
$\text{mass = }{\displaystyle \frac{\text{58}\text{.5 x 0}\text{.125}}{1}}$
mass = 7.32 g
From this we can conclude that the number of moles and grams of NaCl present in 250 mL of a 0.5 M NaCl solution is 0.125 and 7.32 respectively.

Therefore, the correct answer is option (A).

Note: Always remember to convert the volume of the solution into litres before substituting into the formula for molarity. This will help avoid confusion as well as errors.