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Relative mass formula, atomic mass, and empirical

formula

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

More examples

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)

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

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

- 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

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

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

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

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

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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