Moles Calculations

relative atomic mass (Ar)

the relative mass of its atoms compared to the mass of carbon → 12 atoms

relative formula mass (Mr)

calculated using a compound’s formula and the relative masses of the atoms it contains

How to calculate Mr

• work out how many atoms of each element there are in the chemical formula

• add together the Ar values for all the atoms of each element present

Percentage mass

(atomic mass of element present / Mr of compound) x 100 = % mass of element

law of conservation of mass

• during chemical reactions ( and changes of state) no atoms are created/destroyed

• the atoms are rearranged

• the total mass of chemicals before and after a reaction stays the same

open systems

• substances can move between reactions and surroundings

• reactions often appear to change mass

• if gas is produced the mass will appear to decrease as the mass from this gas is lost to surroundings in air

• if a gas is used the mass will appear to increase as the mass from this gas is gained from the surrounding air in the products

moles

unit which tells you the amount of a substance (mol)

represents the number of particles rather than mass of a substance

Avogadro’s constant

6 × 10²³

formula for number of moles

mass of substance (g) / relative formula mass

limiting reagent

• the substance that is completely used up when a reaction is complete

• the theoretical yield (amount) of product is based on the amount of the limiting quality that is available for the reaction

percentage yield formula

[actual yield (g) / theoretical yield (g) ] x 100

atom economy formula

[Mr of desired product / Mr of all products] x 100

formula for number of moles of gas

volume of gas in sample (dm³) / 24 dm³