Additional Chemistry Calculations. Relative atomic and Formula Masses
Relative atomic and Formula Masses
The mass of an atom is too small to deal with in real terms, so we use ‘relative’ masses – Carbon is given a mass of 12, and everything else is compared with it and given a mass, e.g. Oxygen is ‘heavier’, so its relative mass is 16.
Relative formula mass can be found by adding up the relative atomic masses of each element in a compound.
E.g. Carbon Dioxide (CO2)
Carbon has a relative atomic mass of 12
Oxygen has a relative atomic mass of 16
The relative formula mass of Carbon Dioxide is therefore:
12 + (16 x 2) = 44
Because saying ‘relative formula (or atomic!) mass in grams’ is a bit clumsy, we simply say ‘moles’ instead. This means that 1 mole of Carbon Dioxide is 44 grams, or 44g. Simple!
The relative atomic mass can be found by looking at the periodic table, It is always the larger of the two numbers.
x 2 because it’s “O2”
Percentage of an element in a compound
We can use the relative atomic mass (Ar) of elements and the relative formula mass (Mr) of compounds to find out the percentage composition of different elements.
E.g. What percentage mass of white Magnesium Oxide is actually Magnesium, and how much is Oxygen?
24 + 16 = 40
X 100% = 60% is Magnesium, so 40% must be Oxygen!
Work out the formula mass of the compound
Convert this into grams
Work out the percentage by using this equation:
Formula of a compound from its percentage composition
We can also do this backwards! If we know the percentage composition of a compound we can work out the ratio of atoms. This is known as the Empirical Formula. Sometimes this is the same as the molecular formula, but not always (e.g. water has an empirical and molecular formula of H2O. Hydrogen peroxide's empirical formula is HO, but it’s molecular formula is H2O2.
E.g. If 9g of Aluminium react with 35.5g of Chlorine, what is the empirical formula of the compound formed?
Divide the mass of each element by its relative atomic mass to find out the number of moles reacted
Create a ratio and simplify if necessary
Write a formula based on the ratio
= 1/3 moles of Aluminium atoms
= 1 mole of Chlorine atoms
Al : Cl
1/3 : 1
1 : 3
Masses of reactants and products
This is an important calculation when we want to know how much of each reactant to react together. For example, sodium hydroxide reacts with chlorine gas to make bleach. If we have too much Chlorine, some will be wasted. Too little and not all of the sodium hydroxide will react.
2NaOH + Cl2 NaOCl + NaCl + H2O
How much Chlorine gas should we bubble through 100g of Sodium Hydroxide to make Bleach?
23 + 16 + 1 = 40g is one mole of NaOH
2. We have 100g in our reaction so…
100 = 2.5 moles
3. The chemical equation tells us that we need 2 moles of Sodium Hydroxide (2NaOH) for every mole of Chlorine (Cl2).
So we need: 2.5 = 1.25 moles of Chlorine
4. 35.5 x 2 = 71g is one mole of Cl2
So we need 1.25 x 71 = 88.75g of Chlorine to react with 100g of Sodium Hydroxide.
Work out the mass of one mole of Sodium Hydroxide
Calculate how many moles you have in your reaction
Work out how many moles of Chlorine you need
Convert this into a mass for Chlorine
Rather than talk about the yield of a chemical reaction in terms of mass (grams, tonnes etc.) we can talk about the percentage yield. This gives us an idea of the amount of product that the reaction really makes, compared to what it could possibly make under perfect conditions. There are many reasons why we don’t make 100% every time, such as:
Using this reaction “A + B C”, it was found that in perfect conditions, scientists could make 2.5g of C. However, when they tried it out, they only made 1.5. What is the percentage yield of this reaction?
The higher the percentage yield and atom economy, the better the reactions are for the Earth’s resources, as there’s less waste!
Amount of product produced
Maximum amount of product possible
x 100% = 60% percentage yield