Relative Molecular Mass
NOTE: The same method is used to find relative molecular mass and relative formula mass
To find the relative molecular mass of a substance, add together the atomic masses of all the atoms in the molecule.
CH4 contains 1 carbon atom and 4 hydrogen atoms.
The relative atomic mass of carbon is 12 and of hydrogen is 1
The relative molecular mass is therefore 12 + 1 + 1 + 1 + 1 = 16
Number of Moles in a Substance
To find the number of moles in a substance:
Number of moles = mass(g) / mass of 1 mole
Find the number of moles in 2g of H2O
1. find the mass of 1 mole: 1 + 1 + 16 = 18g
2. divide the mass you have (2g) by the mass of one mole (18g): 2/18 = 0.1111
so there are 0.1111 moles of H2O in 2g
To find the empirical formula from a molecular formula, just cancel down the numbers of atoms to their simplest form. E.g. H2O2 would have an empirical formula of HO.
To find the empirical formula of a substance when given the mass of the elements:
- Find the number of moles of each element (mass/mass of 1 mole)
- Divide the results of this by the smallest one (so if there are 2 elements and one has 1 mole but the other has 2 moles, you divide by 1)
- These numbers are the numbers of atoms present in the empirical formula.
To find the percentage of a substance made up by one element:
Mass of the element you want to calculate / Molecular mass of entire molecule * 100
Calculating Masses of Substances from an Equation
E.g. Calculate the mass of oxygen which will react exactly with 0.64g sulfur dioxide according to the equation:
2SO2 + O2 --> 2SO3
1. MAKE SURE THE EQUATION IS BALANCED BEFORE BEGINNING
2. Work out the number of moles of sulfur dioxide. (0.64 / 32 + 16 + 16 = 0.01 mole)
3. Find the reaction ratio from the equation. (2 moles of sulfur dioxide react with 1 mole of oxygen so the ratio is 2:1)
4. Use the ratio to find the number of moles of oxygen needed. (0.01 / 2 = 0.005 moles of oxygen)
5. Find the mass of 0.005 moles of oxygen. (0.005 * (16 + 16) = 0.16g)
ANSWER = 0.16g oxygen will react with 0.64g sulfur dioxide.
To calculate the percentage yield from an experiment:
% yield = actual yield / theoretical yield * 100