Summary BTEC applied science/forensic science unit 1 chemistry: balancing equations

Balancing Equations
Relative atomic mass - Because atoms are so small, the relative atomic mass is the
average of the masses of the isotopes compared to an atom of carbon-12. (Ar)
Relative formula mass - Found by adding the Ar of all the atoms in the molecule of a
compound. (Mr)

● H2O
● Two hydrogen atoms + one oxygen atom
● Mr = (2 x 1) + 16
● Mr = 18

● The balanced chemical equations for a reaction shows us how much of each
chemical s involved in the reaction
Reacting mass calculations
NaOH + HCl -> NaCl + H2O
Q: How much NaCl will be produced from 80g of NaOH?
Step 1) Find molecular mass
NaoH 23 + 16 + 1 = 40
NaCl 23 + 35.5 =
Step 2) How many moles?

mass
Moles =
mass per mole

80
=
40 NaOH produces NaCl at 1:1
= 2 moles of NaOH ratio. So 1 mole of NaOH
2 x 58.5 = 117g creates 1 mole of NaCl



Amount of substance and the mole
Mole - The amount of any substance containing the same number of particles as there are
carbon atoms in exactly 12g of carbon-12
Avogadro constant - NA = 6.02 x 1023 mol-1

m
n = M r so m = n x Mr n = number of moles

m = mass in g
Mr = molar mass in g mol-1
● E.g ‘how many moles in 60g of carbon?
60
● n=
12
● = 5mol