Question

The molarity of a 300ml solution is 0.75M. the amount of solute present in it is ___ gm (molecular mass of solute = 58)
A.13.05 g
B.12.35 g
C.13.33 g
D.12.33 g

Answer 1

Molarity and Molality are two different concepts, where molarity is related to the volume of the solution and molality is related to mass of the solvent.
Formula Used:
Molarity = $\dfrac{{number{\text{ }}of{\text{ }}moles{\text{ }}of{\text{ }}solute}}{{volume{\text{ }}of{\text{ }}solution{\text{ }}\left( {in{\text{ }}litres} \right)}}$

Total mass = (molecular mass) (number of moles of substance present)

Complete step by step answer:
Molarity of a solution is basically the measure of the amount of solute present in the solution. It can be defined in a mathematical relation as the ratio of the numbers of moles of the solute present in the solution to the volume of the solution. It must be noted that the volume of the solution must be denoted in litres.
Hence, we can write this in the form of an equation as:
Molarity = $\dfrac{{number{\text{ }}of{\text{ }}moles{\text{ }}of{\text{ }}solute}}{{volume{\text{ }}of{\text{ }}solution{\text{ }}\left( {in{\text{ }}litres} \right)}}$
In the problem given to us, the data that has been provided is;
Molarity = 0.75M, and the volume of solute = 300ml = 0.3 L
Hence, substituting these values in the above formula we get,
0.75 = $\dfrac{{number{\text{ }}of{\text{ }}moles{\text{ }}of{\text{ }}solute}}{{0.3}}$
$\therefore number{\text{ }}of{\text{ }}moles{\text{ }}of{\text{ }}solute = 0.75 \times 0.3$= 0.225 moles
Now to calculate the mass of the solute, we can use the following formula:
Total mass = (molecular mass) (number of moles of substance present)
= (58) (0.225)
= 13.05 g

Hence, Option A is the correct option.

Note:
The volume of the solution must be converted to litres before proceeding with the calculation. Failing to do so would result in an error of the factor of ${10^3}$