Convert mass and molar concentrations and solve simple dilution calculations.
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Method and worked examples
Molar concentration is amount of solute per solution volume: c=Vn. Use litres when c is in mol L−1; 250mL is 0.250L. The volume is the final solution volume, not the volume of water initially added. Mass concentration is Vm, commonly gL−1. Convert mass concentration to molar concentration by dividing by molar mass in gmol−1.
During a dilution with no reaction or loss, the solute amount is unchanged. Therefore c1V1=c2V2, using consistent volume units. V2 is the final total volume, not the volume of water added. Read the requested entity carefully: 0.100 mol L−1 CaCl2 supplies 0.200 mol L−1 chloride ions if completely dissolved and volume changes from dissociation are already included in the solution volume.
Example 1
Find c for 0.0500 mol NaCl in 250mL solution.
V=0.250L; c=0.2500.0500=0.200 mol L−1.
Example 2
20.0mL of 0.500 mol L−1 solution is diluted to 100.0mL. Find c2.
c2=V2c1V1=0.500×100.020.0=0.100 mol L−1.
Watch out:It means the final solution volume is 100mL, including the original solution.
Concentration and dilution practice | Year 11 Chemistry | Ascent Arcade