1. Calculations using moles 2
Concentration of solutions

2. Objectives:
To be able to calculate the concentration of a solution
To understand why excess of one reactant in a chemical reaction
is necessary
To be able to explain the term ‘limiting reactant’
To use moles in a calculation of theoretical yield

3. Many reactants are in the form of solutions, eg hydrochloric acid etc
The amount of substance dissolved in the solvent is called
the concentration of the solution
The concentration of a solution is sometimes given in g/dm3
For example, a concentration of 150g/dm3 means that there are
150g of solute dissolved in every dm3 (litre) of solvent.

4. Mass of solute (g)
Concentration (g/dm3 ) =
Volume of solution (dm3 )
1 dm3 = 1000cm3

5. Example:
A solution has a concentration of 4.2 g/dm3 . Calculate the mass
of solute dissolved in 250 cm3 of solution.

6. Many solutions used in chemical reactions have amount of solute
measured in moles.
A 1M (molar) solution means that the molar mass of the solute is
dissolved in 1dm3 of solvent
How many grams of HCl are dissolved in 1dm3 of water
to make a 2M solution of HCl?

7. Number of moles
Concentration mol/dm3 =
Volume (dm3)
The concentration of HCl is g/dm3
Work out the concentration in mol/dm3

8. Limiting reactants
When we react magnesium
with hydrochloric acid,
eventually no more bubbles
of hydrogen gas are produced.
This is because one of the
reactants has been used up.
Usually we use excess hydrochloric acid and the magnesium is used up.
The reactant that is used up is called the limiting reactant.

9. Using excess hydrochloric acid means that we know that all the magnesium has been used up and we can calculate how much product we should get.

10. What mass of zinc chloride would we get if 1g of zinc is reacted
with excess hydrochloric acid.
Zn HCl ZnCl H2
How many moles of Zinc in 1g?
No of moles Zn = 1/65
No of moles Zn : no of moles of ZnCl2
1:1
No of moles of ZnCl2 = 1/65
Mass of zinc chloride produced =

11. From these sorts of calculations we can compare the mass of
product that we actually get when we do the experiment to
what we should theoretically get.
Why might these masses not be the same?