Monday, 15 June 2020
Empirical Formula
1. Explain the difference between empirical and
molecular formulae
2. Carry out calculations:
• to find empirical formula from data giving
composition by mass or percentage by mass
• to find molecular formula from the empirical
formula and relative molecular mass.
Specification
Reference:
3.1.2.4
Skills
Maths (MS)
• MS 0.2 - Use ratios, fractions and percentages).
Explain the difference between empirical and molecular formulae
Carry out calculations:
to find empirical formula from data
to find molecular formula from the empirical formula & RMM
15 June 2020
Key Words:
Calculating Empirical Formula
You can use information about reacting masses to calculate
the formula of a compound. Here is an example:
3.2g of sulphur reacts with 3.2g oxygen to produce
6.4g of sulphur oxide.
What is the formula of the oxide?
Use the fact that the Ar of sulfur is 32 and the Ar of
oxygen is 16
Explain the difference between empirical and molecular formulae
Carry out calculations:
to find empirical formula from data
to find molecular formula from the empirical formula & RMM
15 June 2020
Key Words:
Step
Action
S
O
1
find masses
3.2
3.2
2
look up given Ar
values
32
16
3
divide masses by
Ar
0.1
0.2
4
find the ratio
1
2
The calculation tells us that for every 1 sulphur
atom we need 2 oxygen atoms therefore the formula
is: SO2
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Empirical formulae question
• If 9g Aluminium reacts with 35.5g Chlorine what is
the empirical formula of the compound formed?
step
action
1
find masses
2
look up Ar
values
3
divide masses
by Ar
4
find the ratio
Al
Cl
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Empirical formula question
• If 9g Aluminium reacts with 35.5g Chlorine what is
the empirical formula of the compound formed?
step
action
Al
Cl
1
find masses
9
35.5
2
look up Ar
values
27
35.5
3
divide masses
by Ar
0.33
1
1
3
4
find the ratio
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Empirical formulae question
• If 9g Aluminium reacts with 35.5g Chlorine what is
the empirical formula of the compound formed?
AlCl3
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
A sample of the solvent used in one perfume contained 0.60
g of carbon, 0.15 g of hydrogen and 0.40 g of oxygen.
step
action
1
find masses
2
look up given
Ar values
3
divide masses
by Ar
4
find the ratio
C
H
O
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
A sample of the solvent used in one perfume contained 0.60
g of carbon, 0.15 g of hydrogen and 0.40 g of oxygen.
step
action
C
H
O
1
find masses
0.6
0.15
0.4
2
look up given
Ar values
12
1
16
3
divide masses
by Ar
0.05
0.15
0.025
4
find the ratio
2
6
1
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Practical:
Finding the empirical formula of a metal oxide
In this practical, my aim is to achieve the following:
1. Use appropriate apparatus to record the mass of
magnesium
2. Set up apparatus correctly
3. Safely and carefully handle solids- including
flammable
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Practical:
Finding the empirical formula of a metal oxide
1. Fill in the results table and calculate the empirical
formula of magnesium oxide.
2. Calculate % error
Look at the equipment you have used and work out the
error of measurements; some will tell you the error, some
you need to work out (i.e. If a 50cm3 measuring cylinder has
a scale down to 1cm3 then the % error is 0.5cm3)
Error
.
Amount measured
x 100 = % error
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Practical:
Finding the empirical formula of a metal oxide
Evaluations:
At the end of this experiment, I ended up with
These results make sense/ do not make sense because….
In this practical I learned….
I also learned….
I believe I accomplished the goals of this lab because…
In this lab, I got to practice….
Error analysis:
One source of error may have been….
This would have affected my results because….
Another source of error was…..
It would have affected my results because….
If I could do this again, I would….
Know How to find the Ar or Mr of a substance.
Grade C
Describe What each of the calculations show
Grade B
Calculate yields, empirical formula and reacting masses
Grade A
15 June, 2020
Ar = Atomic mass (element)
Mr = Formula mass
(compound)
Key Words: mole, mass, RAM, Empirical formula, yield, balanced equations
Exam question practice
Complete the exam questions based on empirical
formula
You will then peer mark these.
Homework
Complete the practical follow up sheet