Question

How many moles are in sodium chloride in 150mL of a 2mM NaCl solution?

Answer 1

Hint The moles of solute can be calculated using the equation,
$\text{concentration=}\dfrac{\text{Moles of solute}}{\text{Volume of solution}}$
The units of the parameters should be converted into the standard format.

Complete step by step solution:
So in the question, it is given that a solution of sodium chloride is present with a volume of 150mL and the solution has a molarity of 2mM. And from the information given in the question we have to find the moles of solute. So to solve the question let's discuss some basic concepts of solution, its components and various parameters of concentration of the solution. We are very familiar with the term solution from the time we are studying chemistry and let's brush up some concepts about solutions.

Solutions can be defined as the homogeneous mixtures which are formed by two or more substances.
In this mixture i.e. the solution, consists of two components which is solute and a solvent. Solute is the substance which is getting dissolved uniformly in a medium whereas the solvent is the medium in which the solute particles are getting dissolved. We generally refer to solute as solid substances and solvent as liquid substances but other solutions are present with different phases of solute and solvent.

Now as we know how a solution is prepared there are many parameters associated with the solution, one such parameter is concentration. We could find the concentration of the solution formed by relating various parameters like volume of solution, mass of solvent, number of moles etc. The generally used concentration terms are molarity, molality and normality.
So in the question we have to find the number of moles of solute that is dissolved and the data given is the concentration and volume of the solution. So the concentration term expressed here is in terms of molarity. Molarity of a solution can be calculating the moles of solute divided by the volume of solution expressed in liters.
$\text{Molarity=}\dfrac{\text{moles of solute}}{\text{Volume of solution in liter}}$

Rearrange the equation in terms of moles of solute and the equation becomes,
$\text{Moles of solute=Molarity}\times \text{Volume of solution in L}$
Volume of solution is 150mL, it has to be converted into L
The molarity of the solution is 2mM and it has to be converted into Molar.
$\text{Moles of solute=(2 }\\!\\!\times\\!\\!\text{ 1}{{\text{0}}^{\text{-3}}}\text{L) }\\!\\!\times\\!\\!\text{ (150 }\\!\\!\times\\!\\!\text{ 1}{{\text{0}}^{\text{-3}}}\text{mol}\text{.}{{\text{L}}^{\text{-1}}}\text{)=3 }\\!\\!\times\\!\\!\text{ 1}{{\text{0}}^{\text{-4}}}\text{mol}$

Note: To convert mL to L i.e. from a smaller quantity to a bigger quantity the division operation should be carried out.
$\text{1mL=}\dfrac{\text{1}}{\text{1000}}\text{L=1}{{\text{0}}^{\text{-3}}}\text{L}$
To convert millimolar to molar concentration,
$\text{1mM=}\dfrac{\text{1}}{\text{1000}}\text{M=1}{{\text{0}}^{\text{-3}}}\text{M}$
If mass of the solvent in Kg and moles of solute was given, we calculate the concentration term molaity.
$\text{molality=}\dfrac{\text{moles of solute}}{\text{mass of solvent in kg}}$