# chemistry topic 3

?

## RELATIVE FORMULA MASS

• the relative formula mass is the relative atomic masses of all the atoms in the molecular formuala added together
• % mass of an element in a compound= Ar x number of atoms of the element that youre trying to find out / Mr of the compound
• then x100 to find the percentage
1 of 8

## THE MOLE

• the avogadro contstant is 6.02x10^23
• one mole of any substance is just an amount of that substance that contains an avogadro number of particles
• one mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass eg carboon has an Ar of 12 so one mole of carbon weighs exactly 12g
• moles= mass/ Mr       mol is the symbol for the unit moles
2 of 8

## CONSERVATION OF MASS

• in a chemical reaction mass is always conserved meaning no atoms are created or destroyed: no mass is lost or gained so we say mass is conserved during a reaction
• if the mass does change, a gas is usually involved and has most likely escaped
• explanation 1: if the mass increases its because a gas found in the air eg oxygen has reacted with it
• explanation 2: if the mass decreases, one of the products was a gas and the gas has escaped as the reaction wasnt sealed
3 of 8

## LIMITING REACTANTS

• reactions stop when one reactant is used up
• the reaction stop when one of the reactants is used up, any other reactants are in excess
• the amount of product formed is directly proportional to the amount of limiting reactant
4 of 8

## GASES AND SOLUTIONS

• volume of gas= (mass of gas/Mr of gas) x 24
• concentration= mass of solute/volume of solvent
• concentration= number of moles of solute/ volume of solvent
5 of 8

## CONCENTRATION CALCULATIONS

• moles= concentration x volume
• titrations are experiments that let you find the volumes needed for two solutions to react together completely, if you know the concentration of one of the solutions you can use the volumes from the titration experiment along with the reaction equation to find the concentration of the solution
6 of 8

## ATOM ECONOMY

• atom economy- the % of reactants forming useful products
• atom economy=(relative formula mass of desired products/ relative formula mass of all reactants) x100
• high atom ecomony is better for profits and for the enviroments
• reactions with low atom economy use up resources very quickly, and make up a lot of waste which has to be disposed of somehow, theyre also not as profitable
7 of 8

## PERCENTAGE YIELD

• percentage yield= (actual mass/maximum theoretical mass) x100
• yield is always >100% as some reactant or product getz lost on the way
• a low yield comes down to: not all reactants react to make a product (reversible)
• there might be side reactions, they might react with gases in the air
• you lose some product when you seperate it from the reaction mixture
8 of 8