Prestwick Academy Chemistry department

National 5 Unit 1 – Formulae and reaction quantities Notes

Writing chemical formula

Elements

The formula of the vast majority of elements is their chemicalsymbol, e.g. carbon is C and zinc is Zn.

Seven elements exist as diatomic molecules (molecules containing only two atoms), so, their formulae is written as:

SYMBOL2

The elements are hydrogen (H2), oxygen (O2), nitrogen (N2), fluorine (F2), chlorine (Cl2), bromine (Br2) and iodine (I2).

Compounds

Earlier in this Unit you used electron cloud diagrams to work out the formula of covalent compounds e.g. nitrogen hydride:

A compound with the name ending in –ide contains only the 2 elements named, e.g. sodium chloride contains only sodium and chlorine.

Writing formula using a prefix – prefix method

Such compounds often use prefixes in their name to give information about the number of atoms of each element present in the formula, e.g. dinitrogen monoxide contains 2 nitrogen atoms and 1 oxygen atom, giving the formula N2O.

The most common prefixes are:

Mono - 1

Di- 2

Tri- 3

Tetra- 4

Penta- 5

e.g. carbon monoxide (CO), nitrogen dioxide (NO2), sulphur trioxide (SO3) and dinitrogen tetroxide (N2O4)

*Note:When writing the formula of compounds the element’s symbols are used.*

Writing formula – valency method

The formula, for most compounds, depends on the number of bonds each element in the compound forms.

The number of bonds an element can form is called valency, e.g. oxygen can form 2 bonds so has a valency of 2, aluminium can form 3 bonds, so has a valency of 3.

This information should be added to the Periodic Table on page 4 and 6 of the Data booklet.

The formula for most ‘ide’ compounds can be found by following the steps shown below:

  • Write the element’s symbol
  • Place the valency directly beneath the symbol
  • If the valencies of the elements are different ‘SWAP and DROP’. If the valencies of the elements are the same score them out
  • Simplify by dividing by the smallest whole number (1’s can be omitted)
  • Write the formula

e.g. Magnesium chloride Aluminium sulphide Calcium oxide

Symbol:MgClAlS Ca O

Valency:2132 2 2

Swap:1223 2 2 (divide by 2)

Drop: MgCl2 Al2S3CaO

Writing formula – transition metals (roman numerals)

Transition metals, e.g. iron and copper do not have a fixed valency, so how are the formula of their compounds written?

The valency of the metal is given in the compound’s name as a Roman numeral, e.g. copper (I) nitride and iron (III) sulphide:

e.g.Copper(I)nitride Iron(III)sulphide

Symbol: Cu N FeS

Valency: 1 3 32

Swap: 3 1 23

Drop: Cu3NFe2S3

Writing formula – group/complex ions

Compounds that end with ate or ite contain group ions.The formula for group ions is found on page 8 of the Data booklet. This information should be used to write the formula of the compound. The charge on the ions gives an indication of the valency of the group ion.

e.g. Potassium hydroxideMagnesium phosphate

KOHMgPO4

1+ 1- 2+ 3-

KOH 3 2

Mg3(PO4)2

Writing ionic formula

To write ionic formulae write the formula of both ions, then balance the charge, using brackets, e.g.

  • Calcium nitrate: Ca2+ NO3-, bracket Ca2+(NO3-)2
  • Copper(I)carbonate: Cu+ CO32-, bracket (Cu+)2CO32-

Writing chemical equations

Chemical equations use formulae instead of words for the reactants and products in a chemical reaction.

The rules for writing chemical equations are:

  • Write the word equation
  • Underneath each substance write the chemical formula (you may need to use swap and drop to work this out)

Writing balanced chemical equation

  • Balance the equation
Example:

Hydrogen combines with oxygen forming hydrogen oxide

  • hydrogen + oxygen ------> hydrogen oxide
  • H2 + O2 ------> H2O
  • 2H2 + O2 ------> 2H2O

Relative formula mass

Atoms have mass. The total mass of the atoms present in a formula is known as the formula mass.The unit of formula mass is atomic mass unit (a.m.u.).

Before the formula mass of a substance can be calculated its formula has to be known.

Examples
  1. Calculate the formula mass of nitrogen

Formula: N2Formula Mass: 2 N

= 2 x 14

= 28 amu

  1. What is the formula mass of copper(I)oxide?

Formula: Cu1 O2 Formula Mass: Cu2O

Cu2O = 2Cu + O

= 2x63.5 + 16

= 143 amu

  1. Calculate the formula mass of calcium nitrate.

Formula: Ca2+ NO3-Formula Mass: Ca(NO3)2

Ca2+(NO3-)2 = Ca + 2N + 6O

= 40 + 2x14 + 6x16

= 164 amu

The mole

The formula mass in grams in Chemistry is so important that it is given a special name – a MOLE.

1 mole of a substance ------> formula mass in grams

The relationship between moles, formula mass(f. mass)and mass is expressed in the triangle

Examples

a. Calculate the number of moles in 32 g of carbon hydride.

mass

Mole = ------Formula: C4 H1 - CH4

F. mass Formula Mass: C + 4H

32 = 12 + 4x1

16 = 16

mole = 2 moles

b. Calculate the mass of 0.5 mole of calcium carbonate.

Mass = mole x f. massFormula: Ca2+ CO32-

= 0.5 x 100Formula: CaCO3

= 50 gF. Mass: Ca + C + 3O

= 40 + 12 + 3x16

= 100

Using balanced chemical equations

Often in Chemistry you will be asked to calculate the mass of a product from a given mass of reactant, e.g. calculate the mass of carbon dioxide formed when 6 g of carbon burns in excess oxygen.

Obviously, to calculate the mass of carbon dioxide you have to use the 6 g of carbon.

The relationship between the reactant (carbon) and the product (carbon dioxide) is shown in a balanced chemical equation:

C + O2 -----> CO2

From the equation the relationship between the carbon and carbon dioxide can be stated in moles:

1 mole C <------> 1 mole CO2

As the relationship is in moles convert the mass of the reactant, carbon, to moles

i.e. moles = mass/f. mass.

The moles of product, carbon dioxide, are now known from the above 1 mole to 1 mole relationship.

Calculate the mass of the product using the formula,

mass = moles x f. mass.

The worked examples below show the steps that should be taken in all similar calculations involving equations.

Worked Examples

1. Calculate the mass of carbon dioxide formed when 6 g of carbon burns in excess oxygen.

C + O2 -----> CO2

1 mole C ------> 1 mole CO2

12g ------> 44g

6g ------> X

12X = 44 x 6

X =22 g

Mass of CO2 formed is 22 g

2. Calculate the mass of water formed when 4 g of carbon hydride burns in excess oxygen forming carbon dioxide and water.

CH4 + 2 O2 ------> CO2 + 2 H2O

1 mole CH4 ------> 2 moles H2O

16g ------> 2x18g

4g ------> X

16X = 36 x 6

X = 9 g

9 g of water would form

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