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Quantitative

Chemistry

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Representing Chemicals

Symbols and formulae

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Chemical Symbols of elements

Each element has a symbol.
Many you can predict from the name of the element.

And some you can’t!

Atom

P

Phosphorus

N

Nitrogen

O

Oxygen

H

Hydrogen

Symbol

Name

O

N

H

P

Ag

Silver

Pb

Lead

Cu

Copper

Na

Sodium

Symbol

Atom

Name

Na

Cu

Ag

Pb

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Chemical formula of elements

Each element has a symbol.

Some elements exist as particular numbers of atoms bonded together.

This fact can be represented in a formula with a number which shows how many atoms.

O

N

H

P

N

Formula

Molecule

Atom

O

P

O₂

N₂

H₂

P₄

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The formula of molecular compounds

Molecular compounds have formulae that show the type and number of atoms that they are made up from.

Water

Carbon dioxide

Methane

Formula

Name

C

H

C

O

H

O

CH₄

CO₂

H₂O

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The formula of ionic compounds

Ionic compounds are giant structures.
There can be any number of ions in an ionic crystal - but always a definite ratio of ions.

Name	Ratio	Formula
Sodium chloride	1:1
Magnesium chloride	1:2
Aluminium chloride	1:3
Aluminium Oxide	2:3

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

Sodium chloride

A 1:1 ratio

NaCl

AlCl₃

Al₂O₃

MgCl₂

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Ions with groups of atoms

Some ions are single atoms with a charge.

Other ions consist of groups of atoms that remain intact throughout most chemical reactions.
E.g. Nitrate and sulphate ions commonly occur in many chemical reactions.

Chloride Cl^-

nitride N^3-

Sulphide S^2-

Cl^-

N^3-

S^2-

nitrate

NO₃^-

Sulphate

SO₄^2-

N

O

O^-

O

S

O

O^-

O

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Use of brackets in formulae

Ions like nitrate and sulphate remain unchanged throughout many reactions.
Because of this we tend to think of the sulphate ion as a “group” rather than a “collection of individual” sulphur and oxygen atoms.
This affects how we write formulae containing them. Aluminium sulphate contains two Al ions and three sulphate ions.
We write it as Al₂(SO₄)₃

Not Al₂S₃O₁₂

Similar rules apply to ions such as nitrate NO₃^-, hydroxide OH^-, etc.

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Use the information to write out the formula for the compound.

1) Calcium bromide

(One calcium ion, two bromide ions)

2) Ethane

(Two carbon atoms, six hydrogen atoms)

3) Sodium oxide

(Two sodium ions, one oxygen ion)

4) Magnesium hydroxide

(One magnesium ion, two hydroxide ions)

5) Calcium nitrate

(One calcium ion, two nitrate ions)

CaBr₂

C₂H₆

Na₂O

Mg(OH)₂

Ca(NO₃)₂

Activity

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The Masses of chemicals

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Atomic Mass of elements

The atoms of each element have a different mass.
Carbon is given a relative atomic mass (RAM) of 12.
The RAM of other atoms compares them with carbon.
Eg. Hydrogen has a mass of only one twelfth that of carbon and so has a RAM of 1.
Below are the RAMs of some other elements.

Element	Symbol	Times as heavy as carbon	R.A.M
Helium	He	one third
Beryllium	Be	three quarters
Molybdenum	Mo	Eight
Krypton	Kr	Seven
Oxygen	O	One and one third
Silver	Ag	Nine
Calcium	Ca	Three and one third

4

9

96

84

16

108

40

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Formula Mass

For a number of reasons it is useful to use something called the formula mass.
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

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Percentage Composition

It is sometimes useful to know how much of a compound is made up of some particular element.
This is called the percentage composition by mass.

% Z = (Number of atoms of Z) x (atomic Mass of Z) Formula Mass of the compound

E.g. % of oxygen in carbon dioxide (Atomic Masses: C=12. O=16) Formula = Number oxygen atoms = Atomic Mass of O = 16 Formula Mass CO₂ =

% oxygen =

CO₂

2

12 +(2x16)=44

2 x 16 / 44 = 72.7%

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Calculate the percentage of oxygen in the compounds shown below

Formula	Atoms of O	Mass of O	Formula Mass	%age Oxygen
MgO	1
K₂O	1
NaOH	1
SO₂	2

32+(2x16)=64

32

23+16+1=40

16

(2x39)+16 =94

16

24+16=40

16

16x100/40=40%

16x100/94=17%

16x100/40=40%

32x100/64=50%

% Z = (Number of atoms of Z) x (atomic Mass of Z) Formula Mass of the compound

Activity

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Nitrogen is a vital ingredient of fertiliser that is needed for healthy leaf growth.
But which of the two fertilisers ammonium nitrate or urea contains most nitrogen?
To answer this we need to calculate what percentage of nitrogen is in each compound

Activity

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Formulae: Ammonium Nitrate NH₄NO₃: Urea CON₂H₄

Formula	Atoms of N	Mass of N	Formula Mass	%age Nitrogen
NH₄NO₃	2	28
CON₂H₄	2	28

28x100 /80 = 35%

28x100 /60 = 46.7%

14+(1x4)+14+(3x16)=80

12+16+(2x14+(4x1)=

60

And so, in terms of % nitrogen urea is a better fertiliser than ammonium nitrate

Atomic masses H=1: C=12: N=14: O=16

Activity

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Formula from Composition by mass.

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Formula Mass

When a new compound is discovered we have to deduce its formula.
This always involves getting data about the masses of elements that are combined together.
What we have to do is work back from this data to calculate the number of atoms of each element and then calculate the ratio.
In order to do this we divide the mass of each atom by its atomic mass.
The calculation is best done in 5 stages:

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We found 3.2g of copper reacted with 0.8g of oxygen. What is the formula of the oxide of copper that was formed? (At. Mass Cu=64: O=16)

Substance	Copper oxide
1. Elements	Cu	O
2. Mass of each element (g)
3. Mass / Atomic Mass
4. Ratio
5. Formula

3.2

0.8

3.2/64 =0.05

0.8/16 =0.05

1:1

CuO

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We found 5.5g of manganese reacted with 3.2g of oxygen. What is the formula of the oxide of manganese formed? (Atomic. Mass Mn=55: O=16)

Substance	Manganese oxide
1. Elements	Mn	O
2. Mass of each element (g)
3. Mass / Atomic Mass
4. Ratio
5. Formula

5.5

3.2

5.5/55 =0.10

3.2/16 =0.20

1:2

MnO₂

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A chloride of silicon was found to have the following % composition by mass: Silicon 16.5%: Chlorine 83.5%

(Atomic. Mass Si=28: Cl=35.5)

Substance	Silicon Chloride
1. Elements	Si	Cl
2. Mass of each element (g per 100g)
3. Mass / Atomic Mass
4. Ratio
5. Formula

16.5

83.5

16.5/28 =0.59

83.5/35.5 =2.35

Cl÷Si = (2.35 ÷ 0.59) = (3.98)

Ratio of Cl:Si =4:1

SiCl₄

Divide biggest by smallest

Activity

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Calculate the formula of the compounds formed when the following masses of elements react completely:

(Atomic. Mass Si=28: Cl=35.5)

Element 1	Element 2	Atomic Masses	Formula
Fe = 5.6g	Cl=106.5g	Fe=56 Cl=35.5
K = 0.78g	Br=1.6g	K=39: Br=80
P=1.55g	Cl=8.8g	P=31: Cl=35.5
C=0.6g	H=0.2g	C=12: H=1
Mg=4.8g	O=3.2g	Mg=24: O=16

FeCl₃

KBr

PCl₅

CH₄

MgO

Activity

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Formula from Charges on ions

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Charges on ions.

Many elements form ions with some definite charge (E.g. Na⁺, Mg²⁺andO^2-).It is often possible to work out the charge using the Periodic Table.
If we know the charges on the ions that make up the compound then we can work out its formula.
This topic is covered in more detail in the Topic on Bonding but a few slides are included here on how to work out the charges on ions and use these to deduce the formula of simple ionic compounds.

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Charges and Metal ions

Metals usually lose electrons to empty this outer shell.
The number of electrons in the outer shell is usually equal to the group number in the Periodic Table.
Eg. Li =Group 1 Mg=Group2 Al=Group3

Mg

2.8.2 ⇨ Mg²⁺

Al

2.8.3 ⇨

Al³⁺

Li

2.1⇨Li⁺

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Charges and non-metal ions

Elements in Groups 4 onwards generally gain electrons and the number of electrons they gain is equal to the Group Number.

Oxygen (Group 6) gains (8-6) =2 electrons to form O^2-
Chlorine (Group 7) gains (8-7)=1 electron to form Cl^-

Cl

O

2.6⇨2.8 O ⇨ O^2-

2.8.7⇨ 2.8.8 Cl ⇨ Cl^-

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Copy out and fill in the Table below showing what charge ions will be formed from the elements listed.

H

He

Li

Na

K

Be

Sc

Ti

Mg

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

Se

Br

Ca

Kr

Al

P

N

O

S

Cl

F

Ne

Ar

Si

B

C

As

Mg

C

Cl

K

Symbol	Li	N	Cl	Ca	K	Al	O	Br	Na
Group No
Charge

1

5

7

2

1

3

6

7

1

1+

3-

1-

2+

1+

3+

2-

1-

1+

1

2

3

4

5

6

7

0

Activity

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The formulae of ionic compounds

This is most quickly done in 5 stages.

Remember the total + and – charges must =zero

Eg. The formula of calcium bromide.

Symbols: Ca Br
Charge on ions 2+ 1-
Need more of Br
Ratio of ions 1 2
Formula CaBr₂

Br

Ca

Br

Ca²⁺

Br^-

2 electrons

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The formulae of ionic compounds

Eg. The formula of aluminium bromide.

Symbols: Al Br
Charge on ions 3+ 1-
Need more of Br
Ratio of ions 1 3
Formula AlBr₃

Br

Al

Br

3 electrons

Al³⁺

Br^-

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The formulae of ionic compounds

Eg. The formula of aluminium oxide.

Symbols: Al O
Charge on ions 3+ 2-
Need more of O
Ratio of ions 2 3 (to give 6 e^-)
Formula Al₂O₃

O

Al

O

Al

2e^-

Al³⁺

O^2-

Al³⁺

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The formulae of ionic compounds

Eg. The formula of magnesium chloride.

Symbols: Mg Cl
Charge on ions
Need more of
Ratio of ions
Formula

2+

1-

Cl

1:2

MgCl₂

Cl

Mg

Cl

1e^-

Cl^-

Mg²⁺

Cl^-

Activity

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The formulae of ionic compounds

Eg. The formula of sodium oxide.

Symbols: Na O
Charge on ions
Need more of
Ratio of ions
Formula

O

Na

1e^-

Na⁺

O^2-

Na⁺

1+

2-

Na

2 : 1

Na₂O

Activity

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Using the method shown on the last few slides, work out the formula of all the ionic compounds that you can make from combinations of the metals and non-metals shown below:

Metals: Li Ca Na Mg Al K

Non-Metals: F O N Br S Cl

Activity

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Representing Chemical reactions:

Equations.

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Word Equations

All equations take the general form:

Reactants  Products

Word equations simply replace “reactants and

products” with the names of the actual reactants and products. E.g

Reactants		Products
Magnesium + oxygen	
Sodium + water	
Magnesium + lead nitrate	
Nitric acid + calcium hydroxide	

Magnesium oxide

Magnesium nitrate + lead

Sodium hydroxide + hydrogen

Water + calcium nitrate

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Write the word equations for the descriptions below.
The copper oxide was added to hot sulphuric acid and it reacted to give a blue solution of copper sulphate and water.

water

+

copper sulphate



sulphuric acid

+

Copper oxide

The magnesium was added to hot sulphuric acid and it reacted to give colourless magnesium sulphate solution plus hydrogen

hydrogen

+

Magnesium sulphate



sulphuric acid

+

Magnesium

Activity

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Write the word equations for the descriptions below.
The methane burned in oxygen and it reacted to give carbon dioxide and water.

water

+

Carbon dioxide



oxygen

+

methane

The copper metal was placed in the silver nitrate solution. The copper slowly disappeared forming blue copper nitrate solution and needles of silver metal seemed to grow from the surface of the copper

silver

+

Copper nitrate



Silver nitrate

+

copper

Activity

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Chemical Equations

Step 1: Write down the word equation.
Step 2: Replace words with the chemical formula .
Step 3: Check that there are equal numbers of each type of atom on both sides of the equation. If not, then balance the equation by using more than one.
Step 4: Write in the state symbols (s), (l), (g), (aq).

2MgO(s)



2Mg(s) +O₂(g)

2MgO



2Mg + O₂

Oxygen doesn’t balance.Need 2 MgO and so need 2 Mg

MgO



Mg + O₂

magnesium oxide



magnesium + oxygen

Products

Reactants

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Step 1: Write down the word equation.
Step 2: Replace words with the chemical formula .
Step 3: Check that there are equal numbers of each type of atom on both sides of the equation. If not, then balance the equation by using more than one.
Step 4: Write in the state symbols (s), (l), (g), (aq).

Reactants		Products
sodium + water		hydrogen + sodium hydroxide
+		+

+		+
+		+

Na

H₂O

H₂

NaOH

2Na

2H₂O

2NaOH

H₂

2Na(s)

2H₂O(l)

H₂(g)

2NaOH(aq)

Hydrogen doesn’t balance.

Use 2 H₂O, NaOH, 2Na

Chemical Equations

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Step 1: Write down the word equation.
Step 2: Replace words with the chemical formula .
Step 3: Check that there are equal numbers of each type of atom on both sides of the equation. If not, then balance the equation by using more than one.
Step 4: Write in the state symbols (s), (l), (g), (aq).

Reactants			Products
magnesium + lead nitrate			magnesium nitrate + lead
+			+

+		+

Mg

Mg(NO₃)₂

Pb

Mg(s)

Pb(NO₃)₂(aq)

Mg(NO₃)₂(aq)

Pb(s)

Already balances.

Just add state symbols

Pb(NO₃)₂

Chemical Equations

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Below are some chemical equations where the formulae are correct but the balancing step has not been done. Write in appropriate coefficients (numbers) to make them balance.

Reactants		Products
AgNO₃(aq) + CaCl₂(aq)		Ca(NO₃)₂(aq) + AgCl(s)
CH₄(g) + O₂(g)		CO₂(g) + H₂O(g)
Mg(s) + Ag₂O(s)		MgO(s) + Ag(s)
NaOH + H₂SO₄(aq)		Na₂SO₄(aq) + H₂O(l)

2

Activity

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Reacting Masses

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Conservation of Mass

New substances are made during chemical reactions
However, the same atoms are present before and after reaction. They have just joined up in different ways.
Because of this the total mass of reactants is always equal to the total mass of products.
This idea is known as the Law of Conservation of Mass.

Reaction

but no

mass change

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There are examples where the mass may seem to change during a reaction.
Eg. In reactions where a gas is given off the mass of the chemicals in the flask will decrease because gas atoms will leave the flask. If we carry the same reaction in a strong sealed container the mass is unchanged.

Mg

HCl

Gas given off.

Mass of chemicals in flask decreases

11.71

Same reaction in sealed container:

No change in mass

Conservation of Mass

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Reacting Mass and formula mass

The formula mass in grams of any substance contains the same number of particles. We call this amount of substance 1 mole.

Atomic Masses: H=1; Mg=24; O=16; C=12; N=14

1 mole of methane molecules

12 + (1x4)

CH₄

1 mole of magnesium oxide

24 + 16

MgO

1 mole of hydrogen molecules

1x2

H₂

1 mole of nitric acid

1+14+(3x16)

HNO₃

Contains

Formula Mass

Symbol

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Reacting Mass and Equations

By using the formula masses in grams ( moles) we can deduce what masses of reactants to use and what mass of products will be formed.

carbon + oxygen  carbon dioxide

C + O₂ CO₂

12 + 2 x 16  12+(2x16)

12g 32g 44g

So we need 32g of oxygen to react with 12g of carbon and 44g of carbon dioxide is formed in the reaction.

Atomic masses: C=12; O=16

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What mass of aluminium and chlorine react together?

aluminium + chlorine  aluminium chloride

2Al + 3Cl₂ 2AlCl₃

2 x 27 + 3 x (2x35.5)  2x (27+(3x35.5)

54g 213g 267g

So 54g of aluminium react with 213g of chlorine to give 267g of aluminium chloride.

Atomic masses: Cl=35.5; Al=27

Activity

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What mass of magnesium and oxygen react together?

magnesium + oxygen 

+ 

Atomic masses: Mg=24; O=16

Magnesium oxide

Mg

O₂

MgO

2

2 x 24

2x16

2x(24+16)

48g

32g

80g

So 48g of magnesium react with 32g of oxygen to give 80g of magnesium oxide.

Activity

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What mass of sodium chloride is formed when sodium hydroxide and hydrochloric acid react together?

Sodium + hydrochloric  + hydroxide + acid

+  +

Atomic masses: Na = 23 O = 16 H = 1 Cl = 35.5

Sodium chloride

NaOH

HCl

NaCl

23+1+16

1+35.5

23+35.5

40g

36.5g

58.5g

So 40g of sodium hydroxide react with 36.5g of hydrochloric acid to give 58.5g of sodium chloride.

H₂O

water

(2x1)+16

18g

Activity

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It is important to go through the process in the correct order to avoid mistakes.

Step 1 Word Equation

Step 2 Replace words with correct formula.

Step 3 Balance the equation.

Step 4 Write in formula masses. Remember: where the equation shows more than 1 molecule to include this in the calculation.

Step 5 Add grams to the numbers.

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Reacting Mass and Scale Factors

We may be able to calculate that 48g of magnesium gives 80g of magnesium oxide – but can we calculate what mass of magnesium oxide we would get from burning 1000g of magnesium? There are 3 extra steps:

Step 1	Will 1000g of Mg give more or less MgO than 48g?
Step 2	I need to scale ? the 48g to 1000g. What scale factor does this give?
Step 3	If 48g Mg gives 80g of MgO What mass does 1000g give?
	Answer

more

up

1000 = 20.83 48

20.83 x 80

1667g

Activity

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Mg + CuSO₄ MgSO₄ + Cu
24 64+32+(4x16) 24+32+(4x16) 64
24g 160g 120g 64g What mass of copper will I get when 2 grams of magnesium is added to excess (more than enough) copper sulphate?

Step 1	Will 2g of Mg give more or less Cu than 24g?
Step 2	I need to scale ? the 24g to 2g. What scale factor does this give?
Step 3	If 24g Mg gives 64g of Cu What mass does 2g give?
	Answer

less

down

2 = 0.0833 24

0.0833 x 64

5.3 g

Activity

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CaCO₃  CaO + CO₂
40+12+(3x16) 40+16 12+(2x16)
100g 56g 44g
What mass of calcium oxide will I get when 20 grams of limestone is decomposed?

Step 1	Will 20g of CaCO₃ give more or less CaO than 100g?
Step 2	I need to scale ? the 100g to 20g. What scale factor does this give?
Step 3	If 100g CaCo₃ gives 56g of CaO What mass does 20g give?
	Answer

less

down

20 = 0.20 100

0.20 x 56

11.2g

Activity

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Reacting Mass Industrial Processes

Industrial processes use tonnes of reactants not grams.
We can still use equation and formula masses to calculate masses of reactants and products.
We simply swap grams for tonnes.
E.g. What mass of CaO does 200 tonnes of CaCO₃ give?

CaCO₃  CaO + CO₂

100 56 44

So 100 tonnes would give ? tonnes

And 200 tonnes will give

Scale factor =

So mass of CaO formed = ? tonnes =

56

more

200/100 =2

2 x 56

112 tonnes

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Iron is extracted from iron oxide Fe₂O₃
E.g. What mass of Fe does 100 tonnes of Fe₂O₃ give?

Fe₂O₃₊ 3CO  2Fe + 3CO₂

160 84 112 + 132

So 160 tonnes would give ? tonnes

And 100 tonnes will give

Scale factor =

So mass of Fe formed = ? =

112

less

100/160 =0.625

0.625 x 112

70 tonnes

Activity

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Ammonia is made from nitrogen and hydrogen
E.g. What mass of NH₃ is formed when 50 tonnes of N₂ is completely converted to ammonia?

N₂₊ 3H₂  2NH₃

28 6 34

So 28 tonnes would give ? tonnes

And 50 tonnes will give than 28 tonnes

Scale factor =

So mass of NH₃ formed = ? =

34

more

50/28 =1.786

1.786 x 34

60.7 tonnes

Activity

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Na is the symbol for?

Nitrogen
Nickel
Neodynium
Sodium

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Which of these does NOT exist as a diatomic molecule (2 bonded atoms)?

Nitrogen
Oxygen
Calcium
Chlorine

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How many oxygen atoms are represented in the formula Pb(NO₃)₂?

One
Two
Three
Six

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What is the formula mass of MgCl₂?

Mg=24 Cl=35.5

59.5
83.5
95
119

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What is the formula mass of Mg(OH)₂?

Mg=24 O=16 H = 1

41
42
57
58

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What is the percentage nitrogen in ammonium sulphate (NH₄)₂SO₄?

21%
42%
63%
84%

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What is the formula of a compound containing 1.4g nitrogen and 3.2g of oxygen? (N=14 O=16)

N₂O
NO
NO₂
N₂O₃

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What is the formula of a compound containing 6.5g zinc and 1.6g oxygen?

(Zn=65 O=16)

ZnO
Zn₂O₃
ZnO₂
Zn₂O

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What is the formula of a compound formed between Cr³⁺ ions and O^2- ions?

CrO
Cr₂O₃
CrO₂
Cr₃O₂

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What is the formula of a compound formed between Cr³⁺ ions and OH^- ions?

CrOH₃
Cr₃OH
Cr(OH)₃
Cr₂OH₃

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What is the word equation for the reaction described below?

A small piece of strontium metal was added to water. It fizzed giving off hydrogen gas leaving an alkaline solution of strontium hydroxide.

Strontium + water  hydrogen + strontium hydride
Strontium + water  oxygen + strontium hydroxide
Strontium + water  hydrogen + strontium hydrate
Strontium + water  hydrogen + strontium hydroxide

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What numbers a - d are needed to balance the equation?

Strontium + water  hydrogen + strontium hydroxide

a Sr + b H₂O  c H₂+ d Sr(OH)₂

a=1 b=1 c=1 d=1
a=1 b=2 c=1 d=1
a=1 b=1 c=2 d=1
a=1 b=1 c=1 d=2

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What is the mass of 2 moles of magnesium nitrate Mg(NO₃)₂?

86g
134g
148g
296g

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How many moles of iron atoms is 280g of iron? (Fe=56)

One mole
Two moles
Four moles
Five moles

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When iron rusts it forms the iron oxide Fe₂O_3.What mass of oxygen reacts with 112g of iron? (Fe=56 O=16)

1g
16g
48g
168g

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Hydrogen reacts with chlorine to form hydrogen chloride HCl.

H₂ + Cl₂  2HCl

What mass of hydrogen chloride will be obtained from 4g of hydrogen gas?

(H=1 Cl=35.5)

36.5g
73g
109.5g
146g