1.7 Quantitative Chemistry

Relative atomic mass (A_r)

mass of the atom compared to carbon-12 isotope (mass of exactly 12), and is weighted mean of mass numbers

Relative formula mass (M_r)

total relative atomic masses of atoms in the formula (ignore coefficient)

Percentage of element in compound

mass of element/ relative formula mass

Mole (mol)

unit for measuring amount of a substance, 6.02×10²³ particles

Mass of 1 mol in g

equivalent to relative formula mass e.g 12g carbon = 1 mol

Mole equation

moles = mass (g)/ M_r

1 tonne

1000 kg

1 kg

1000 g

Molar ratio

in balanced equations this is the coefficient (big number)

Calculating masses from moles

1. find moles of substance given in g using formula

2. use molar ratio to find mol of substance asked

3. convert moles into grams

Limiting reactant

reactant totally used up so it determines amount of product formed

Excess

more than required to react with limiting reactant, left over

Calculating mass of substance formed

1. calculate moles for each reactant

2. divide mole values by molar ratio

3. use limiting reactant (smallest) to find final mass

Theoretical yield

maximum possible amount of product that can be made

Actual yield

amount of product made by carrying out the reaction

Reasons actual yield is less than theoretical

• product is lost when removed from reaction mixture (stuck/ separation)

• unwanted side reactions that use up reactants without forming desired product

• reaction may be reversible so products reform reactants (⇌) and not going to completion

Percentage yield equation

actual yield/ theoretical yield x 100

Why we use percentage yield

measure the efficiency of a reaction

Molecular formula

actual number of atoms of each element present in molecule

Empirical formula

simplest, whole number ratio of atoms of each element in compound

Calculating empirical formula of compound

1. heat compound to constant mass, metals increase (metal oxide)

2. find mass of each element by subracting

3. convert to moles using formula and simplify

Heating to constant mass

heating and weighing, repeating this until two readings are the same

Water of crystallisation

water chemically bonded into the crystal structure

Hydrated

solid crystals contain water of crystallisation

Dehydration

removal of water of crystallisation

Anhydrous

does not contain water of crystallisation

Degree of hydration

number of moles of water of crystallisation chemically bonded in 1 mole of the compound

Calculating degree of water of crystallisation

1. find mass and M_r of each compound

2. calculate their moles

3. divide by smallest number of moles

Determining percentage water of crystallisation

degree hydration * M_r of water/ M_r of compound * 100