1. Using moles and relative formula mass

2. In chemistry, you always have access to at least one piece of information – relative atomic mass This means you can work out relative formula mass too. Where do you find this information?

3. Moles Calculations By the end of the lesson you will be able to....
Rearrange the mole triangle to convert mass to moles and vice versa.
Work out the formula of a substance if you know the percentage masses.

4. To start... What is a mole (description)?
What is the value for a mole?
If we had a mole of bananas, how many bananas would we have?
If we had a mole of carbon atoms, how many atoms would we have?
What is the mass of a mole of carbon atoms?

5. Calculate the number of moles (to 2dp) in...
Mass, Moles and RAM (Mr)
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...
13g of Na
26g of Mg
46g of Ca
0.57 moles
1.08 moles
1.15 moles

6. 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

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

8. Example
CH4 (g) + H2O (g)  CO (g) + 3H2 (g) I have started with 8g of methane and I want to work out how much carbon monoxide I will produce when I react it with steam…… I have two pieces of information from the triangle, what are those pieces?

9. Mass = 8g RAM = 12 + 4 = 16 What can I now work out?

10. Moles = mass/ RAM = 8/ 16 = 0.5 moles Now look at the equation: CH4 (g) + H2O (g)  CO (g) + 3H2 (g) 1 mole 1 mole 1 mole 3 moles

11. Use a ratio
CH4 (g) + H2O (g)  CO (g) + 3H2 (g) 1 mole 1 mole 1 mole 3 moles 1: 1: 1: 3 If I have 0.5 moles of CH4 what number do I have of everything else?

12. Using the number of moles…
I now know that I have 0.5 moles of methane producing 0.5 moles of carbon monoxide. To work out the mass, use the triangle again…

13. Mass = moles x RAM
So mass = 0.5 x RAM of CO = 0.5 x 28 = 14g

14. Step-by-step guide List the information you have i.e. mass, moles, RAM
Work out the number of moles for the reactant or product that you can (i.e. the one that you have 2 pieces of information for)
Look at the equation, what is the ratio in which things react?
Multiply or divide the number of moles you have calculated according to the ratio
Then take moles and RAM and use the equation triangle to work out the mass

15. Calculating Empirical Formula
Empirical formula is just the simplest whole number ratio of atoms in a compound eg: Benzene, C6H6 has the empirical formula CH. If we know the percentages of each element present then we can calculate the empirical formula.

16. Calculating Empirical Formula
Example, A compound contains 75% carbon and 25% hydrogen. What is its empirical formula? C H
Amount 75 25
Convert to moles ( /Mr)
/12 = 6.25
/1 = 25
Calculate mole ratio (divide by smallest number)
6.25/ 6.25
25/6.25
= 1
= 4
Empirical formula H4

17. Have a go at calculating the
empirical formula...
An oxide of carbon contains 27% carbon and 73% oxygen. What is its empirical formula?
Fluorspar is made of calcium and fluorine. If 51% is calcium, calculate the empirical formula.
1.68g of iron is combined with 0.48g of oxygen. What is the empirical formula of the new compound ?

18. Moles Calculations Can you....
Rearrange the mole triangle to convert mass to moles and vice versa.
Work out the formula of a substance if you know the percentage masses.
Can you....