State symbols
s - solid
l - liquid
g - gas
aq - aqueous (the substance can be dissolved in water, a solution)
Relative atomic Mass (Ar)
Atomic number (top number) of the element
E.g. Hydrogen = 1
Oxygen = 16
Relative Formula Mass (Mr)
To calculate Mr of a compound we have to add up the relative atomic masses of all the atoms that make up that compound
E.g. H2O
Hydrogen = 1x2=2
Oxygen = 16
Ans: 2+16=18
Calculate percentage by mass of an element within a compound
Formula: (Ar x no. of atoms/Mr) x 100
E.g. Calculate hydrogen within H2O
(1x2/1x2+16) x 100
(2/18) x 100
0.11 x 100 =11
Calculate the number of moles/mass
Moles = mass/Mr (or Ar)
Mass= number of moles x Mr
E.g. Calculate 4g of magnesium oxide, MgO
= 4/24+16 =4/40
= 0.1 moles
E.g. Calculate the mass of 4 moles of calcium oxide, MgO
= 0.1 x 24+16 =0.1 x 40
= 4g
Reacting mass calculation
See book
Limiting Reagents
See book
Concentration
The mass of a substance dissolved in a given volume of water. The units are grams per decimetre cubed
(g/dm ³ )
Formula: Concentration = Mass/(volume/1000)
E.g. What is the concentration (in g/dm ³ ) of 5g copper sulphate dissolved in 100cm ³ of distilled water
=5/(100/1000) =5/0.1
= 50g/dm ³
Convert cm ³ to dm ³
Cm ³ /1000 =0.001 cm³
dm³ x 1000 =cm ³
