3.4 Using concentrations of solutions in mol/dm³
- The concentration of a solution can be measured in mol/dm³.
- The amount in moles of solute or the mass in grams of solute in a
given volume of solution can be calculated from its concentration in mol/dm³.
- If the volumes of two solutions that react completely are known and
the concentration of one solution is known, the concentration of the
other solution can be calculated.
Calculating concentration
The concentration of a solution in mol/dm³ is calculated using the formula:
Concentration (mol/dm³) = Amount of solute (moles) / Volume of solution (dm³)
To relate this to mass, first calculate the number of moles using:
Moles = Mass of solute (g) / Molar mass (g/mol)
So, the concentration can also be found by:
Concentration (mol/dm³) = [Mass of solute (g) / Molar mass (g/mol)] / Volume of solution (dm³)
This shows that the concentration depends on both the mass of the solute dissolved and the total volume of the solution.
Worked example
A student dissolves 5.8 g of sodium chloride (NaCl) in enough water to make 0.5 dm³
of solution. Calculate the concentration of the solution in mol/dm³.
Step 1: Calculate the number of moles of NaCl.
Molar mass of NaCl = 23 (Na) + 35.5 (Cl) = 58.5 g/mol
Moles of NaCl = Mass / Molar mass = 5.8 g / 58.5 g/mol = 0.099 mol
Step 2: Calculate the concentration.
Concentration (mol/dm³) = Moles / Volume = 0.099 mol / 0.5 dm³ = 0.198 mol/dm³
Answer: The concentration of the solution is 0.198 mol/dm³.