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calculations involving masses (magnesium oxide practical (heat to constant…
calculations involving masses
find Mr of a compound given the RAMs of the elements
empirical formula
divide mass (given in q) by Mr to find moles of the substances
find the ratio of the moles
the ratio will be the number of atoms for each element e.g. 2:3 means Fe2O3
magnesium oxide practical
heat to constant mass. until all the mg has turned white
heat the crucible but lift the lid constantly to let oxygen in
reweigh the crucible+mg
minus crucible mass to find the new MgO mass
then weigh magnesium+crucible
minus Mg from MgO to find mass of oxygen
weigh some pure magnesium and weigh the crucible+lid.
use these masses and O and Mg Mr to work out mole ratio and then empirical formula
conservation of mass
mass of products = mass of reactants
total mass of a closed system before and after doesnt change as no atoms are destroyed/created
shown by precipitation reaction
copper sulfate solution reacts with sodium hydroxide
forms insoluble copper hydroxide and soluble sodium sulfate
mass balance underneath reading wont change - mass is conserved
mass changes
mass increases
at least one of the reactants is a gas thats found in air e.g. oxygen
and the products are solids, liquids or aqueous
e.g. metal and oxygen to metal oxide
mass decreases
at least one of the reactants are soldis/liquids/aqueous
and at least one of the products is a gas
e.g. metal carbonate to metal oxide and CO2
moles
one mole of atoms or molecules of any substance will have a mass in grams equal to the Mr for that substance
carbon has an Mr of 12 so one mole of carbon weighs 12g
nitrogen gas, N2 has an Mr of 28 so 1 mole of N2 weighs 28g
there are 6.02 x 10^23 atoms/molecules/ions in a mole
mass = Mr x moles
mass = conc. x vol.
limiting reactants
any reaction stops when one of the reactants are used up
limiting reactant is the one is the one thats used up in the reaction
product formed directly proportional to amount of limiting reactant used. this is because if you add more of the limiting reactant there will be more reactanr particles to take part in the reaction
work out amount of product from the limiting reactant
write balanced equation then work out Mr's of reactant/product.
calculate moles of substance you know the mass of
using balanced equation, work out moles for the other substance. then calculate mass using the Mr
balancing equations using reacting masses