Concentration measures how much solute is packed into a solution
Definition
Solution
A mixture formed when a solute dissolves in a solvent.
Definition
Solute
The substance dissolved in a solvent to form a solution.
- Many reactions take place in solution, so it helps to describe how much dissolved substance a solution contains.
- A solute dissolves in a solvent to make the solution.
- The concentration tells you how much solute is present in a given volume of that solution.
Key Idea
- More solute in the same volume makes a more concentrated solution.
- The same solute spread through a larger volume makes a more dilute solution.
Mass concentration is measured in grams per cubic decimetre
Definition
Concentration
The mass of solute present per unit volume of solution, usually measured in g/dm3\text{g}/\text{dm}^3g/dm3 for this calculation.
- Concentration in g/dm3\text{g}/\text{dm}^3g/dm3 is the mass of solute divided by the volume of solution, or c=mVc = \dfrac{m}{V}c=Vm.
- Here mmm is the mass of solute in grams and VVV is the volume of solution in dm3\text{dm}^3dm3.
Example
Dissolving 20 g20\ \text{g}20 g of salt in 0.5 dm30.5\ \text{dm}^30.5 dm3 of water gives a concentration of 200.5=40 g/dm3\dfrac{20}{0.5} = 40\ \text{g}/\text{dm}^30.520=40 g/dm3.
Convert the volume from cm3cm^3cm3 to dm3dm^3dm3 before you start
- One cubic decimetre equals 1000 cm31000\ \text{cm}^31000 cm3, so divide a volume in cm3\text{cm}^3cm3 by 100010001000 to get dm3\text{dm}^3dm3.
- For example, 250 cm3250\ \text{cm}^3250 cm3 is 2501000=0.25 dm3\dfrac{250}{1000} = 0.25\ \text{dm}^31000250=0.25 dm3.
Exam technique
- Convert every volume to dm3\text{dm}^3dm3 first, as using cm3\text{cm}^3cm3 makes the concentration a thousand times too small.
- Always write the unit g/dm3\text{g}/\text{dm}^3g/dm3 with a mass concentration answer.
Finding the mass of solute in a solution
- Rearranging the formula gives the mass of solute as m=c×Vm = c \times Vm=c×V.
- For 250 cm3250\ \text{cm}^3250 cm3 of a 40 g/dm340\ \text{g}/\text{dm}^340 g/dm3 solution, first write the volume as 0.25 dm30.25\ \text{dm}^30.25 dm3.
- The mass of solute is 40×0.25=10 g40 \times 0.25 = 10\ \text{g}40×0.25=10 g.
Self review
- What does the concentration of a solution tell you?
- How do you convert a volume in cm3\text{cm}^3cm3 into dm3\text{dm}^3dm3?
- Find the concentration when 30 g30\ \text{g}30 g of solute is dissolved to make 2 dm32\ \text{dm}^32 dm3 of solution.
- What mass of solute is in 500 cm3500\ \text{cm}^3500 cm3 of a 20 g/dm320\ \text{g}/\text{dm}^320 g/dm3 solution?
