C3 - quantitative chemistry

123 - relative formula mass

• relative atomic mass is written as Ar

• relative formula mass is written as Mr

• you can use the periodic table to find the relative atomic masses - this is the bottom number, comprised of protons and neutrons

• compounds have relative formula masses, this is all the relative atomic masses in the compound added together

• if there are 2 atoms in a compound (Cl2), the relative atomic mass of Chlorine will be x by 2, then added to the relative atomic mass of the other element in the compound

• example - what is the relative formula mass of MgCl2

• Ar of Mg = 24

• Ar of Cl 35.5

• 24 + 35.5 × 2 = 95

• Mr of MgCl2 = 95

• to calculate the percentage mass in a compound, use the formula - Ar x number of atoms of that element / Mr of the compound x 100

• example - find the percentage mass of sodium in sodium carbonate, Na2CO3

• Ar of Na = 23, Ar of C = 12, Ar of O = 16

• Ar x number of atoms of that element - 23 × 2 = 46

• Mr of the compound - 23 × 2 + 12 + 16 × 3 = 106

• Percentage mass of sodium = 46 / 106 × 100 = 43%

124 - the mole

• avogadro’s constant = 6.02 × 10 to the power of 23

• one mole of a substance is the amount of a substance that contains 6.02 × 10 to the power of 23 particles

• one mole of a substance is the same as it’s relative formula/atomic mass

• e.g carbon has an Ar of 16, so one mole of carbon is 16g

• e.g nitrogen gas (N2) has a Mr of 28 (2 × 14), so one mole of N2 weighs 28g

• to calculate the number of moles, use: moles = mass in g / Mr

• mol is the symbol for moles

• the equation triangle is - mass = moles x Mr

125 - conservation of mass

• during a chemical reaction, mass is conserved, meaning no atoms are created or destroyed

• the total Mr of all the reactant should equal the Mr of the products

• you can do this in a balanced symbol equation to find out if mass is conserved

• if there is a change in mass, there is usually a gas involved

• explanation 1: if there’s an increase in mass, one of the reactants is a gas found in air and all the products are solid, liquid or aqueous - before the reaction the gas is in air not contained in the reaction vessel, the mass will increase when the gas gets contained in the reaction vessel

• explanation 2: if there’s a decrease in mass, one of the products is a gas and all the reactants are solid, liquid or aqueous - before the reaction, all the reactants are contained in the reaction vessel, after the reaction, if it isn’t enclosed gas will be produced and escape from the reaction vessel

126 - the mole and equations

• the big numbers in front of elements in a reaction tell you how many moles are used or formed, if there’s no number then there’s one mole

• 2Mg = 2 moles of magnesium

• H = 1 mole of hydrogen

• 1) work out the Mr of each of the substances in the equation

• 2) use the Mr and the masses given in the question to calculate how many moles of each substances of each substance reacted or were produced using moles = mass / Mr

• 3) divide each of the moles by the smallest mole to get the ratio

• 4) the numbers should be whole, for example, if you calculate 2.0 for Zinc, there are two moles of zinc used and produced in the equation, so write it out as 2Zn

127 - limiting reactants

• a limiting reactant is one that gets used up before the rest

• reaction stops when all of one of the reactants is used up, any others that are left are in excess, they’re added in excess to ensure the other reactant is used up

• limiting reactant used is directly proportional to the amount of product formed

• the more reactant you use, the more reactant particles avaliable for collision

• when magnesium carbonate/MgCO3 is reacted with hydrochloric acid, you know the reaction is taking place when gas is being produced, the fizzing slows down and the reaction eventually stops

• to calculate the mass of product formed by using the mass of the limited reactant and the balanced symbol for the reaction:

• 1) write out the balanced equation

• 2) work out the Mr of the reactant and product specified in the question - to find Mr - look at bottom number on periodic table and x by how many atoms of that elements are used in the equation

• 3) only find the Mr of the limiting reactant, the reactant with the bigger mass has been used in excess, so only find both the Mr’s but only use the smaller one when finding the mole

• 4) find out how many moles there are of the substances you know - to find moles use mass / Mr - the mass will be given in the question

• 5) look at the ratio in the equation, e.g - 4Al + 3o2 = 2Alo3 - four moles of aluminium react to make 2 moles of aluminium oxide so the ratio is 4:2

• 6) apply the ratio - if you calculated that there are 5 mols of aluminium in 135g, then those 5 mols will react to make 2.5 mols of aluminium oxide - half of 4 is two, half of 5 is 2.5

• 7) calculate the mass (g) of 2.5 mols of aluminium oxide - mass = moles x Mr

128 - concentrations of solutions

• concentration is different to strength - concentration is how much of a solution there is in another liquid (how watered down it is)

• the more solute there is dissolved in a solution, the more concentrated it is

• concentration is measured in g/dm3

• concentration = mass of solute (g) / volume of solvent (dm3)

• dm3 - cm3 = divide by 1000

• equation triangle to find either mass of solute of volume of solvent - mass = conc x vol