Concentration of solutions
Solutions are formed when solutes dissolve in solvents. If the number of moles of solute and the volume of solvent used is known, the concentration of the solution can be calculated.
The concentration of a solution is measured in moles per litre (mol l-1) and can be calculated using this formula triangle:
Using the formula triangle gives three useful formulae:
\(concentration = \frac{{no.\,of\,moles}}{{volume}}\)
\(volume = \frac{{no.\,of\,moles}}{{concentration}}\)
\(no.\,of\,moles = concentration \times volume\)
Example one
Calculate the concentration of the solution formed when 0.25 moles of potassium nitrate are dissolved in 500 cm3 of water.
The formula we need to use, given by covering up concentration is:
\(concentration = \frac{{no.\,of\,moles}}{{volume}}\)
\(= \frac{{0.25}}{{0.5}}\)
\(=0.5\,mol\,l_{}^{ - 1}\)
Remember that the volume always has to be changed into litres.
Example two
Calculate the number of moles in 200cm3 of 0.5 mol l-1 sodium hydroxide (NaOH) solution.
\(number\,of\,moles = concentration \times volume\,(in\,litres)\)
\(\begin{array}{l} = 0.5 \times 0.2\\ = 0.1\,moles \end{array}\)
Example three
Calculate the volume of sodium hydroxide used in 1 mol l-1 solution containing 2 mol of solute.
\(volume = \frac{{number\,of\,moles}}{{concentration}}\)
\(= \frac{2}{1}\)
\(= 2l\)