Relative formula mass and empirical

Relative formula mass and empirical

formulaObjectives:

• To know how to write chemical formula.

• To understand the meaning of relative

atomic mass.

• To understand what an empirical formula

is.

• To be able to calculate relative formula

mass.

• To be able to calculate empirical formula.

Starter

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SC9a Masses and empirical formulae

Quick Quiz

1 What is the formula of water?

2 What is the formula of sodium chloride?

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Quick Quiz

1 What is the formula of water?

H2O

2 What is the formula of sodium chloride?

NaCl



Quick Quiz

3 What is the formula of carbon dioxide?

4 The formula of magnesium chloride is MgCl2.

What is the ratio of magnesium ions to chloride

ions?



Quick Quiz

3 What is the formula of carbon dioxide?

CO2

4 The formula of magnesium chloride is MgCl2.

What is the ratio of magnesium ions to chloride

ions?

1:2



Quick Quiz

5 The formula of sulfuric acid is H2SO4. How many

atoms of each element are in the formula?

6 The formula of calcium nitrate is Ca(NO3)2. How

many calcium, nitrogen and oxygen atoms are in

the formula?



Quick Quiz

5 The formula of sulfuric acid is H2SO4. How many

atoms of each element are in the formula?

H = 2, S = 1, O = 4

6 The formula of calcium nitrate is Ca(NO3)2. How

many calcium, nitrogen and oxygen atoms are in

the formula?

Ca = 1, N = 2, O = 6



Quick Quiz

7 There are two numbers alongside chlorine in the

periodic table, 17 and 35.5. What does the

number 17 represent?

8 What does the number 35.5 represent?



Quick Quiz

7 There are two numbers alongside chlorine in the

periodic table, 17 and 35.5. What does the

number 17 represent?

atomic number

8 What does the number 35.5 represent?

relative atomic mass (mass number)



formulaObjectives:



atomic mass.

• To understand what an empirical formula

is.


mass.

• To be able to calculate empirical formula.

• Why is it called relative atomic mass?

The relative mass of an object is the mass of that

object compared to the mass of another object.

For example, relative

to the girl, the tiger is

four times as heavy.

We say it has a

relative mass of 4.

Atoms are measured

relative to 1/12th the

mass of a 12C atom.

12C atoms have a relative

mass of 12.

1H atoms are twelve

times lighter so have a

relative mass of 1.

SC9a Relative formula masses and empirical formulae


The relative formula mass (Mr) of a substance is the sum

of the relative atomic masses (Ar) of all the atoms or ions in

its formula.

relative atomic mass : Ar (O)

relative formula mass : Mr(HNO3)

Worked example

The formula of nitric acid is HNO3.

This means that it contains 1 hydrogen, 1 nitrogen and

3 oxygens.

Mr of HNO3 = Ar(H) + Ar(N) + 3 × Ar(O)

= 1 + 14 + (3 × 16)

= 63 Notice that

Mrs have

no units

Relative Formula Mass• How will we calculate this?• To calculate this we simply add together

the atomic masses of all the atoms shown in the formula. (N=14; H=1; Na=23; O=16; Mg=24; Ca=40)

Substance Formula Formula Mass

Ammonia NH3

Sodium oxide Na2O

Magnesium hydroxide Mg(OH)2

Calcium nitrate Ca(NO3)2

14 + (3x1)=17

(2x23) + 16 =62

24+ 2(16+1)=58

40+ 2(14+(3x16))=164

Calculate the relative formula mass of the

following substances.

a) F2

b) Fe

c) H2SO4

d) Al2O3

e) Al(NO3)3

f) (NH4)2SO4

g) CuCO3

h) AgNO3

i) NH4NO3

RAMs

F=19

Fe=56

H=1

S=32

O=16

Al=27

N=14

Cu=63.5

C=12

Ag=108

Calculate the relative formula mass of the

following substances.

a) F2

b) Fe

c) H2SO4

d) Al2O3

e) Al(NO3)3

f) (NH4)2SO4

g) CuCO3

h) AgNO3

i) NH4NO3

Answers

a) 38

b) 56

c) 98

d) 102

e) 213

f) 132

g) 123.5

h) 170

i) 80

RAMs

F=19

Fe=56

H=1

S=32

O=16

Al=27

N=14

Cu=63.5

C=12

Ag=108


formulaObjectives:



atomic mass.

• To understand what an empirical

formula is.


mass.

• To be able to calculate empirical

formula.

Empirical formula

• The simplest whole number ratio of atoms

in a compound

eg. H2O2 empirical formula is

C3H6 empirical formula is

H O

CH2

Molecular formula

(true formula of compound)



Worked example

Aluminium bromide is formed from 0.51 g of aluminium and

4.49 g of bromine. Calculate the empirical formula of

aluminium bromide.

Relative atomic masses:

Al = 27 Br = 80

Set your working out in columns and explain each step.



Al Br

Mass (g) 0.51 4.49

You can round

numbers that

are close to a

whole number,

but if it is .5,

double both

numbers

Don’t forget

to write the

formula from

the ratio

÷ by RAM

÷ by smallest number

Simplest whole

number ratio

Empirical formula

= 0.0189 = 0.0561

= 1 = 2.97

1 3

AlBr3



Question

A sample of sodium phosphate contains 3.45 g of sodium,

1.55 g of phosphorus and 3.20 g of oxygen. Calculate the

empirical formula of sodium phosphate.


O = 16 Na = 23 P = 31



Na P O

Mass (g) 3.45 1.55 3.20

Answer

= 0.15 = 0.05 = 0.20

= 3 = 1 = 4

Na3PO4

÷ RAM

÷ 0.05

Empirical formula



Question

An oxide of phosphorus contains 3.49 g of phosphorus and

4.51 g of oxygen. Calculate the empirical formula of this

phosphorus oxide.


O = 16 P = 31



P O

Answer

Divide the mass of

each element by the Ar

Divide the answers by

the smallest number…

…to find the

simplest ratio

Simplest whole

number ratio

Empirical formula

= 0.113 = 0.282

= 1 = 2.5

2 5

P2O5

Plenary

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Question

Calculate the relative formula mass of:

a zinc sulfate, ZnSO4

b iron(III) oxide, Fe2O3

c aluminium carbonate, Al2(CO3)3.


C = 12 S = 32

O = 16 Fe = 56

Al = 27 Zn = 65



a Mr of ZnSO4 = Ar(Zn) + Ar(S) + 4 × Ar(O)

= 65 + 32 + (4 × 16)

= 161

b Mr of Fe2O3 = 2 × Ar(Fe) + 3 × Ar(O)

= (2 × 56) + (3 × 16)

= 160

c Mr of Al2(CO3)3 = 2 × Ar(Al) + 3 × Ar(C) + 9 × Ar(O)

= (2 × 27) + (3 × 12) + (9 × 16)

= 234

When part of a

formula is in a

bracket,

multiply

everything

inside the

bracket by the

number outside

Answer


Quick Quiz

9 Sodium chloride has the formula NaCl. The

relative atomic mass of sodium is 23 and that of

chlorine is 35.5. What is the relative formula

mass of NaCl?

58.5

10 A water molecule has the formula H2O. The

relative atomic mass of hydrogen is 1 and that of

oxygen is 16. What is the relative formula mass

of a molecule of water?

18


• We calculate the empirical formula of a

compound by looking at the masses of

elements that react together to form the

compound.

• Analysis of 20.1g of iron bromide showed

that it contained 3.8g of iron and 16.3g of

bromine. What is its empirical formula?

Fe : Br

Mass (g) 3.8 : 16.3

÷RAM 3.8 : 16.3

56 80

= 0.068 : 0.204

÷smaller 0.068 : 0.204

0.068 0.068

= 1 : 3

Empirical formula: FeBr3

•

1 Calculate the empirical formula of the following

substances:

a water, if a sample contains 0.6 g of hydrogen

and 4.8 g of oxygen (H=1, O=16)

H : O

Mass (g) 0.6 : 4.8

÷RAM 0.6 : 4.8

1 16

= 0.6 : 0.3

÷smaller 0.6 : 0.3

0.3 0.3

= 2 : 1

Empirical formula: H2O

•

1 Calculate the empirical formula of the following

substances:

b lithium carbonate, if a sample contains 2.8 g of

lithium, 2.4 g of carbon and 9.6 g of oxygen

(Li=7,C=12, O=16)

Li : C : O

Mass (g) 2.8 : 2.4 : 9.6

÷RAM 2.8 : 2.4 : 9.6

7 12 16

= 0.4 : 0.2 : 0.6

÷smaller 0.4 : 0.2 : 0.6

0.2 0.2 0.2

= 2 : 1 : 3

Empirical formula: Li2CO3

Relative formula mass and empirical

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