Download 9.4 Empirical and molecular formula

Date:Tuesday, 15 June
2021
Unit 9: The Mole
Outcomes:
By the end of the
lesson you should be
able to do these things.
What should I do if I
cannot do one or more
of these things?
Keywords
For every lesson there
will be a list of key
words here.
•
•
•
•
•
Topic 1: Measuring Matter
Topic 2: Mass and the Mole
Topic 3: Moles of compounds
Topic 4: Empirical and molecular formulas
Topic 5: Formulas of Hydrates
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
Empirical and Molecular Formulas
Starter:
The molecular formula for succinic acid is C4H6O4.
The empirical formula for succinic acid is C2H3O2.
What do you think the difference is between molecular
and empirical formula?
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
What is the difference between molecular
formula and empirical formula?
The molecular formula specifies the actual number of atoms of
each element in one molecular formula unit of a substance.
e.g. Ethane is C2H6
Ethane: C2H6
The empirical formula of a compound gives you the simplest whole
number ratio of the elements in a compounds.
e.g. ethane is CH3.
Sometimes the empirical and the molecular formulae of a
compound are the same, like for water, H2O.
Water: H2O
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
TASK: Match the correct empirical formula
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
Percent Composition from experimental data
• The percent by mass of a compound shows us how much of each
type of element are in a given sample of a substance.
• It can be calculated by dividing the mass of the element by the
mass of the compound and multiplying by a 100.
• Percent by mass (element) = mass of element x 100
mass of compound
Worked example: A 100 g sample of a compound contains 55g of
Element X and 45g of Element Y. Calculate the percent composition
of this sample.
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
Percent Composition from chemical formula
• Percent composition can also be determined from chemical
formula.
• To do this, assume that you have 1 mol of the compound – so we
can use the molar mass of the compound.
• Percent by mass = mass of element (in 1 mol of compound) x 100
molar mass of compound
Worked example: Page 315, Example 10.
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
Calculating percentage by mass – example 2
What percentage by mass of nitrogen is in ammonia (NH3)? (atomic mass: H = 1, N = 14)
Step 1: Work out the molar mass of NH3.
Molar mass
of NH3
= 1 nitrogen atom + 3 hydrogen atoms
= (1 x 14) + (3 x 1)
= 17
Step 2: Work out the percentage by mass of nitrogen.
Atomic mass x number of atoms
% of nitrogen in NH3
=
Molar mass of compound
(14 x 1)
=
17
= 82%
x 100
x 100
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
Task: Calculate the percent by mass of…
• Na in NaOH
• S in CuSO4
• O in Ba(OH)2
• Cl in CaCl2
• Ca in CaCO3
• Al in Al2O3
• N in AgNO3
Lesson 2 ---Contd
starter
Empirical Formula –
Rhyme to Remember
“Percent to mass; Mass to mole; Divide
by small; Multiply ‘til whole”
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
Empirical Formula
• Percent composition or masses of the elements in a
given mass of compound can be used to determine
the formula for the compound.
• If percent composition is given, assume the total
mass of the compound is 100g…
• For example: The percent composition of a compound is
40.05% S and 59.95% O.
• In 100g of this compound, we would have 40.05 g S and
59.95 g of O.
• We then can convert these masses into moles…
• This provides a mole ratio – divide by the smallest number
if they are not whole numbers.
• You now have the empirical formula.
Date:Tuesday, 15 June 2021
Outcomes:
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
Task 1 ( 5 min)
• What is the empirical formula of a compound that
contains 60g of Carbon, 13g of hydrogen and 27g of
oxygen?
Solve this questions now !
Date:Tuesday, 15 June 2021
Outcomes:
Molecular Formula
1. Calculate percent
composition from
experimental data and
from the chemical
formula.
• To determine molecular formula, we must compare
the molar mass of the compound (experimentally
determined) with the mass of the empirical formula.
2. Determine the empirical
formula from percent
composition and from
mass data.
• Experimentally determined molar mass = n
3. Determine the
molecular formula by
comparing the molar
mass.
Keywords
Empirical formula
Molecular formula
Molar mass
Percent by mass
mass of empirical formula
• Molecular formula = n(empirical formula)