Mole and concentration of solutions

Mole and concentration of solutions

The concentration of a solution is the measure of the amount of a solute that is dissolved in a fixed volume (1dm³ or 1000cm³) of the solution.

The concentration of a solution is divided into two:

a. Mass concentration

b. Molar concentration.

Class work

calculate the molar concentration of a solution of hydrochloric acid (HCl), which has a mass concentration of 9.125gdm^-3. ( H=1, Cl = 35.5)

a. Mass concentration: This is the mass of solute that is dissolved in 1dm3 of the solution.
The unit of mass concentration is grams per cubic decimetre of the solution (gdm-3).

Mathematically,

 Mass concentration = $\frac{mass}{1dm^{-3}}$  

b. Molar concentration: Is the number of moles of solute that is dissolved in 1dm³ of the solution.

The unit for molar concentration is moles of the solute per cubic decimetre of the solution (moldm^-3).

 $\ \frac{\ Number\ of\ moles}{1dm^{-3}}$  

Mathematically,
Molar concentration =

Example: A solution of sodium hydroxide (NaOH) contains 20.0g of sodium hydroxide dissolved in 1dm3 of solution. Calculate the mass concentration and molar concentration of the solution.

Solution
Mass = 20g

a. Mass concentration =

 $\frac{Mass}{1dm^3}$  = $\frac{20.0}{1}\ =\ 20gdm^{-3}$  

b. Molar concentration = $\ \frac{number\ ofmoles}{1dm}$  

Number of moles = $\ \frac{\ mass\ concentration}{molar\ mass}$  

mass concentration =  $20gdm^{-3}$ 

Molar mass of NaOH = 23 + 16 + 1 = $40gmol^{-1}$  

Number of moles = $\frac{20}{40}\ =\ 0.5\ mol$  

Therefore, Molar concentration =  $\frac{0.5}{1}\ =\ 0.5moldm^{-3}$