Question

How many moles of $ HBr $ are present in $ 18{\text{mL}} $ of a $ 0.400{\text{M}} $ solution?

Answer 1

Hint: Molarity or the molar concentration of a solution is a measure of the concentration of a chemical species in a solution. We know that the SI unit for molarity is moles per litre. Mathematically the molarity can be expressed as the ratio of number of moles of the solute to the volume of solution in litres.

Formulas used: We will be using the formula, $ c = \dfrac{n}{V} $
where $ c $ is the molar concentration or the molarity while $ n $ is the number of moles of the solute present in the solution and $ V $ is the volume of the solution in litres.

Complete step by step solution:
We know that molarity is a measure of concentration of a chemical substance in a solution. The molarity of a solution can tell you about the composition of the solution and changing the amount of solute of the volume of the solvent can have very drastic changes in the molar concentration.
It is also closely related to other quantities that measure concentration of a solution like number concentration, mass concentration, mole fraction, mass fraction or the molality of the solution. So, using the mathematical formula for molarity we know that,
 $ c = \dfrac{n}{V} $
We know from the problem that the molarity of $ HBr $ solution is $ 0.400{\text{M}} $ and the solution is found in a volume of $ 18{\text{mL}} $ . Now to find the number of moles of solute we use the formula for molarity such that the result yields the number of moles of solute,
 $ n = c \times V $
Substituting the known values, we get,
 $ n = 0.400 \times \left( {18 \times {{10}^{ - 3}}} \right) $
Since the volume has to be converted to litres.
 $ n = 7.20 \times {10^{ - 3}} $
The number of moles of $ HBr $ is $ n = 7.20 \times {10^{ - 3}}{\text{moles}} $ .

Note:
If you ought to check the answer for the problem then, we can check it using the units, we are required to find the amount of substance in moles and our answer is in moles, thus the answer can be considered right. However, check for the units of volume and do the conversion as required.