2.6 - Concentration of Solutions
Concentration in mol/dm3
- Concentration is also measured as the amount of solute, in moles, in each cubic decimetre of solution (mol/dm3).
- More solute in the same volume makes a solution more concentrated; more solvent for the same solute makes it more dilute.
Measuring volume in dm3
- 1 dm3 = 1,000 cm3 = 1 litre
- To convert cm3 to dm3, divide by 1,000.
Formula for concentration in mol/dm3
Where:
- Concentration is in moles per cubic decimetre (mol/dm3)
- Amount of solute is in moles (mol)
- Volume of solution is in cubic decimetres (dm3)
Worked example - Concentration in mol/dm3 from a mass
5.00 g of sodium hydroxide, NaOH, is dissolved in water and the solution is made up to a total volume of 300 cm3. Calculate the concentration of the solution in mol/dm3.
Step 1 - Identify the values
- Mass of sodium hydroxide = 5.00 g
- Volume of solution = 300 cm3
Step 2 - Convert the mass to an amount in moles
Relative atomic masses (periodic table): Na = 23, O = 16, H = 1, so of NaOH = 23 + 16 + 1 = 40. Amount = mass ÷ :
amount of NaOH = 5.00 ÷ 40 = 0.125 mol
Step 3 - Convert the volume to dm3
1,000 cm3 = 1 dm3:
300 cm3 = 300 ÷ 1,000 = 0.300 dm3
Step 4 - Substitution and calculation
Concentration = 0.125 ÷ 0.300 = 0.41666... mol/dm3
Step 5 - Round the answer
The mass and volume are both given to three significant figures, so:
Concentration = 0.417 mol/dm3
TermDefinition
ConcentrationA measure of how much of a substance is present in a given volume, usually how much solute is dissolved in a given volume of solution.