1. Using moles in calculations

2. Objectives :
to be able to work out how many moles there are in a certain mass
of a substance
To be able to work out how many grams of a substance is present,
knowing the number of moles present
To be able to calculate reacting masses and products in a reaction
To be able to write a balance equation from the reacting masses
and products of a reaction

3. You have learnt:
A mole of any substance contains 6.02 X 1023 particles
A mole of an atom is the relative atomic mass of the
expressed in grams
A mole of a molecule is the relative formula mass expressed
in grams

4. Learning Check
Prozac, C17H18F3NO, is a widely used antidepressant that inhibits the uptake of serotonin by the brain. Find its molar mass.

5. If we have 13g of Sodium (Na), how many moles of sodium
do we have?
We know that the mass of 1mole of Na is
23g
So the number of moles in 13g is
13 / 23
=0.57
(If a bag of apples has a mass of 2000g and each apple has a mass
of 200g, how many apples do we have?

6. Calculate the number of moles (to 2dp) in...
To calculate the number of moles we need to know the mass and the Mr (RAM):
Moles = Mass Mr
Calculate the number of moles (to 2dp) in...
26g of Mg
46g of Ca

7. Mass, Moles and RAM (Mr) 14g 42g 9.6g
To calculate the mass of a substance when we know how many moles there are...
Mass = Moles x Mr
Calculate the mass of...
2 moles of Li
3.5 moles of C
0.3 moles of S
14g
42g
9.6g

8. Mass, Moles and RAM (Mr)
Mass
Moles
RAM

9. The same principle can be used to find the number of moles of a
molecule that is present

10. Learning Check!
The artificial sweetener aspartame (Nutra-Sweet) formula C14H18N2O5 is used to sweeten diet foods, coffee and soft drinks. How many moles of aspartame are present in 225 g of aspartame?

11. Calculations using equations
2Mg + O MgO
What does this balanced equation tell us?
Two atoms of Mg react with 1 molecule of oxygen to produce
two molecules of magnesium oxide.
What happens if we start with X1023 (2 moles) magnesium atoms?
They react with 6.02 X 1023 (1 mole) of oxygen molecules
To produce X (2 moles) of magnesium oxide

12. How do we know how many grams to weigh out?
How do we know how many grams of magnesium oxide we will get?
All our calculations will need to bypass a calculation on mole ratio
Ratio of number of moles of magnesium: number of moles of MgO
=2:2 (1:1)

13. Eg Calculate the mass of MgO made from 6g of Mg?
Step 1:
How many moles of Mg are in 6g of Mg?
Mass of one mole of Mg = 24g
No of moles Mg = 6/24
= 0.25
Step 2:
How many moles of MgO are made?
0.25
Step 3:
What mass of MgO is made?
Mass of one mole of MgO = 40g
Mass of MgO = 0.25 X 40
= 10g

14. Eg. Calculate the mass of Mg needed to make 2g MgO
Step 1:
How many moles are there in 2g MgO
Number of moles = 2/30
=0.067
Step 2:
How many moles of Mg are needed?
0.067
Step 3:
Mass of Mg needed = X 24
=1.6g

15. You need to start with a correctly balanced equation for the reaction
You need to work in moles and molar ratios in your calculations

16. During a reactions, 56g of nitrogen reacts with hydrogen to form ammonia (NH3). What is the mass of hydrogen that reacts?
Balanced equation
N H NH3 3 2
No of moles of nitrogen = mass / molar mass
= 56 /28
= 2
No of moles of hydrogen needed = 2 X 3 = 6
Mass of hydrogen needed = 6 x2 = 12g

17. We can also use information on the masses of reactants and products
to balance equations.
The key to doing this is also moles

18. Aluminium oxide, Al2O3 produces aluminium, Al and oxygen O2
If 204g of aluminium oxide produce 108g of Al, write down a full
balanced equation.
Molar mass of aluminium oxide = (2 X 27) + (3 X 16)
=102g
Number of moles of aluminium oxide = 204/102
= 2
Number of moles of Al produced = 108/ 27
= 4
Molar ratio = 2:4
2Al2O3
4Al + 3 ? O2