Relative mass formula, atomic mass, and empirical formula

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Relative mass formula atomic mass and empirical
formula
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Relative formula mass Mr
The relative formula mass of a compound is
blatantly the relative atomic masses of all the
elements in the compound added together.
E.g. water H2O
Therefore Mr for water 16 (2x1) 18
Work out Mr for the following compounds

• HCl
• NaOH
• MgCl2
• H2SO4
• K2CO3

H1 Cl35 so Mr 36 Na23 O16 H1 so Mr
40 Mg24 Cl35 so Mr 24(2x35) 94 H1 S32
O16 so Mr (2x1)32(4x16) 98 K39 C12
O16 so Mr (2x39)12(3x16) 138
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More examples
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Relative atomic mass

• The mass of an isotopic element relative to
  Carbon-12.
• Example chlorine occurs in isotope forms Cl-35
  (75.5) and Cl-37 (24.5)
• Relative atomic mass
• ((75.5x35)(24.5x37))/(75.524.5)35.5
• Try this neon-20 (90.9) neon-21 (0.3) and
  neon-22 (8.8)

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Calculating percentage mass
If you can work out Mr then this bit is easy
Calculate the percentage mass of magnesium in
magnesium oxide MgO
Ar for magnesium 24 Ar for oxygen 16 Mr for
magnesium oxide 24 16 40 Therefore
percentage mass 24/40 x 100 60

• Calculate the percentage mass of the following
• Hydrogen in hydrochloric acid HCl
• Potassium in potassium chloride KCl
• Calcium in calcium chloride CaCl2
• Oxygen in water H2O

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Calculating the mass of a product
E.g. what mass of magnesium oxide is produced
when 60g of magnesium is burned in air
Step 2 WORK OUT the relative formula masses
(Mr) 2Mg 2 x 24 48 2MgO 2 x
(2416) 80

• Step 3 LEARN and APPLY the following 3 points
• 48g of Mg makes 80g of MgO
• 1g of Mg makes 80/48 1.66g of MgO
• 60g of Mg makes 1.66 x 60 100g of MgO

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• Work out Mr 2H2O 2 x ((2x1)16) 36
  2H2 2x2 4
• 36g of water produces 4g of hydrogen
• So 1g of water produces 4/36 0.11g of hydrogen
• 6g of water will produce (4/36) x 6 0.66g of
  hydrogen

Mr 2Ca 2x40 80
2CaO 2 x (4016) 112 80g produces 112g so
10g produces (112/80) x 10 14g of CaO
Mr 2Al2O3 2x((2x27)(3x16)) 204 4Al
4x27 108 204g produces 108g so 100g produces
(108/204) x 100 52.9g of Al2O3
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Another method
Try using this equation
Mass of product IN GRAMMES
4
6g
36
So mass of product (4/36) x 6g 0.66g of
hydrogen
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Calculating the volume of a product
At normal temperature and pressure the Relative
Formula Mass (Mr) of a gas will occupy a volume
of 24 litres
e.g. 2g of H2 has a volume of 24 litres 32g of O2
has a volume of 24 litres 44g of CO2 has a volume
of 24 litres etc

• On the previous page we said that the MASS of
  hydrogen produced was 0.66g
• 2g of hydrogen (H2) will occupy 24 litres (from
  the red box above)
• So 0.66g will occupy 0.66/2 x 24 8 litres

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Example questions

• What volume of hydrogen is produced when 18g of
  water is electrolysed
• 2H20 2H2 O2
• Marble chips are made of calcium carbonate
  (CaCO3). What volume of carbon dioxide will be
  released when 500g of CaCO3 is reacted with
  dilute hydrochloric acid
• CaCO3 2HCl CaCl2 H2O CO2
• In your coursework you reacted magnesium with
  hydrochloric acid. What volume of hydrogen would
  be produced if you reacted 1g of magnesium with
  excess acid
• Mg 2HCl MgCl2 H2

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Empirical formulae
Empirical formulae is simply a way of showing how
many atoms are in a molecule (like a chemical
formula). For example CaO CaCO3 H20 and KMnO4
are all empirical formulae. Heres how to work
them out
A classic exam question Find the simplest
formula of 2.24g of iron reacting with 0.96g of
oxygen.
Step 1 Divide both masses by the relative atomic
mass For iron 2.24/56 0.04 For oxygen 0.96/16
0.06 Step 2 Write this as a ratio and
simplify 0.040.06 is equivalent to 23 Step 3
Write the formula 2 iron atoms for 3 oxygen
atoms means the formula is Fe2O3
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Example questions

• Find the empirical formula of magnesium oxide
  which contains 48g of magnesium and 32g of
  oxygen.
• Find the empirical formula of a compound that
  contains 42g of nitrogen and 9g of hydrogen.
• Find the empirical formula of a compound
  containing 20g of calcium 6g of carbon and 24g
  of oxygen.

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