1.21 use the state symbols (s), (l), (g) and (aq) in chemical equations to represent solids, liquids, gases and aqueous solutions respectively
1.22 understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation
1.23 calculate empirical and molecular formulae from experimental data
1.24 calculate reacting masses using experimental data and chemical equations
1.26 carry out mole calculations using volumes and molar concentrations (in the post 'Mole calculations')
1.20 write word equations and balanced chemical equations to represent the reactions studied in this specification
You should be able to do this by now, so just practice:
Metal + acid à salt + hydrogen
Magnesium + hydrochloric acid àmagnesium chloride + hydrogen
Zinc + hydrochloric acid à
Magnesium + nitric acid à
+ à zinc sulphate + hydrogen
+ à iron chloride + hydrogen
Now try them again using symbols, remember, hydrogen is a diatomic molecule!
Metal + water à metal hydroxide + hydrogen
Only very reactive metals react with water in this way, others react with steam (water vapour) to form metal oxides:
Metal + steam à metal oxide + hydrogen
Complete these word equations:
Sodium + water à
Magnesium + water à
Magnesium + steam à
Iron + steam à
Complete these symbol equations:
1. Na + H2O à NaOH + H2
2. Mg + H2O(g) à MgO +
3. Mg + H2O(l) à Mg(OH)2 + H2
There are many more equations out there obviously, just remember the common formulas and balance the
equation!
equation!
Rules for naming compounds:
- If a compound contains only 2 elements, it will end in 'ide'.
- If it contains only a metal and a non-metal, the non-metal ends in 'ide'.
- If it contains only 2 non-metals, the name will say how many atoms of each there are.
Common number prefixes:
1- Mono
2- Di
3- Tri
4- Tetra
5- Penta
6- Hexa
4. If the compound contains a metal, non-metal and oxygen, it will end in ‘ate’
5. If it contains the OH group it is called a hydroxide
Beware that there are some with common names that don’t follow the rules! E.g. H2O should be dihydrogen monoxide, but it is commonly known as water! Also, e.g.
- NH3(g)-ammonia
- HCl(aq)-hydrochloric acid
- H2SO4(aq)-sulphuric acid
- HNO3(aq)-nitric acid
- CH4(g)-methane
Working out the name of a compound from its formulae:
Formula | Name |
CO2 | Carbon dioxide |
SO2 | |
SO3 | |
HCl | |
H2S | |
Li3N | |
NaCl | |
NaNO3 | |
Na2CO3 | |
Cu(NO3)2 | |
FeCl2 | |
Fe(OH)3 | |
BaCl2 | |
Fe2(SO4)3 | |
ZnCl2 | |
AlCl3 | |
Ca(NO3)2 | |
PbO | |
PbO2 | |
Ca(OH)2 | |
CaCl2 | |
ZnSO4 | |
AgNO3 | |
NaHCO3 | |
(NH4)2SO4 | |
KAt | |
NH4Cl |
Obtaining empirical formula from experimental data, (taken from my past work):
- cut out a strip of magnesium, approximately 10cm long
- weigh an empty crucible (without the lid) on a 2 d.p balance 10.66 g
- add the magnesium and weigh again 21 g
- heat the magnesium in the crucible over a Bunsen burner until the reaction is complete
- when the crucible is cooled, reweigh it. 10.99 g
Element | Mg | O |
Mass (g) | 0.21 | 0.12 |
Mr | 24 | 16 |
Number of moles (m/Mr) | 0.0.00875 | 0.0075 |
Ratio of moles (divide by smallest) | ≈1.2 x5 | 1 x5 |
à6 | à5 | |
Empirical formula | Mg6O5≈MgO | |
Possible causes of error:
- incomplete reaction
- impurities in magnesium
- some magnesium oxide ash escaped
- inaccurate scales
- some soot on crucible due to yellow flame, adding mass
- some magnesium nitride formed Mg3N2
Empirical formula: the smallest integer ratio of atoms of each element in one molecule of a substance
Molecular formula: the actual number of atoms of each element in one molecule of a substance
One without errors:
e.g. 24g of magnesium was heated in air. The resulting oxide had a mass of 40g.
What do we need to know to work out the empirical formula?
1. The mass of each element in the compound:
The total mass of the oxide is 40g.
24g of this is magnesium, so 16g of it must be oxygen.
2. The number of moles of each element (n=m/Mr):
Magnesium: n=24g/24=1
Oxygen: n=16g/16=1
3. The ratio of elements= 1:1 so the empirical formula is MgO- 1 oxygen atom to every magnesium atom.
You can also lay it out in a table form (you don’t have to draw the table, but have the left column laid out and fill in the missing bits) to keep your working clear:
Example: A hydrocarbon is found to contain 1.2g of carbon and 0.4g of hydrogen.
Element | C | H |
Mass/percentage | 1.2 | 0.4 |
Mr | 12 | 1 |
Number of moles (m/Mr) | 0.1 | 0.4 |
Ratio of moles (divide by smallest) (DBS) | 0.1/0.1=1 | 0.4/0.1=4 |
Empirical formula | CH4 | |
Work out the empirical formulae for the following compounds:
- Ca (40%), C (12%), O (48%)
- Na (32.4%), S (22.5%), O (45.1%0
- Pb (92.8%), O(7.2%)
- Pb (90.66%), O(9.34%)
- C (75%), H (25%)
To work out the molecular formula, if you know the molar mass and the empirical formula, you need to work out the unit mass of the empirical formula, and then work out how many units are in the molecular formula:
e.g. if you worked out the empirical formula of a compound is CH3, the unit mass is :
(1 x 12) + (3 x 1) = 15
If the formula mass of the compound is 30, 30/15=2, there are 2 ‘units’ in it, so 2 lots of CH3.
Therefore the molecular formula of the compound is C2H6.
I like to think of molecular formula as:
MF= n x EF, where n=RFM of MF / RFM of EF
Work out the molecular formula of these compounds:
- C (40%), H (6.67%), O (53.3%), Mr=60
- H-0.5g, S-16g, O-32g, Mr=194
Answers to questions in this post are in:
http://askmichellechemistry.blogspot.com/2012/04/answers-to-questions-on-chemical.html
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