Mole calculations in solutions [Higher tier 鈥 GCSE Chemistry only]
Concentration calculations
You can calculate the amount of a substance in The amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 脳10虏鲁 number of particles). in a solution if you know the volume and concentration. You can also work out the concentration of an acid reacting with an alkali, or vice versa.
Concentration, amount of The dissolved substance in a solution. and volume of Mixture formed by a solute dissolving in a solvent. are linked by this equation:
Concentration in mol/dm3 = amount in mol 梅 volume in dm3
This equation can be rearranged to find the amount of solute or volume of solution.
Divide a volume in cm3 by 1,000 to convert it to a volume in dm3.
Calculating concentration
Question
0.5 mol of solute is dissolved in 250 cm3 of solution. Calculate its concentration.
250 cm3 = 250 梅 1,000 = 0.25 dm3
Concentration = 0.5 梅 0.25 = 2.0 mol/dm3
Calculating amount of solute
Rearranging the equation: amount = concentration 脳 volume
Question
Calculate the amount of solute dissolved in 2 dm3 of a 0.1 mol/dm3 solution.
Amount = 0.1 脳 2 = 0.2 mol
Calculating volume
Rearranging the equation: volume = amount 梅 concentration
Question
Calculate the volume of a 2 mol/dm3 solution that contains 0.5 mol of solute.
Volume = 0.5 梅 2 = 0.25 dm3
Note that 0.25 dm3 is the same as 250 cm3 (0.25 脳 1,000).
Concentrations can also be expressed in grams per decimetre cubed (g/dm3). To convert the two different units of concentration, use the relative formula mass (Mr).