4.4 - Moles I
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what is the relative formula mass/ RFM
the mass of a formula unit of substance
how is RFM calculated
by adding the relative atomic masses of the atoms or ions
what does percentage by mass show
the mass of a substance that comes from a particular element
what is the equation for percentage by mass
%(x) = mass of (x) / RFM x100
what is a mole
the unit of amount of substance
what do moles represent
how many particles of a substance there are, taking into account particles of different substances have different masses
what is the equation for calculating moles
moles = mass(g) / RFM
what is the empirical formula
contains the simplest mole ratio of its elements
how do you calculate the empirical formula
1- convert masses or percentages into moles 2- cancel down the mole ratio 3- write the simplest mole ratio into a formula
what should you draw to calculate the empirical formula
mass, moles, ratio table
what is the molecular formula
contains the actual numbers of atoms in a molecule - like the empirical formula but uncancelled
how do you calculate the molecular formula the quick way for example using the empirical formula CH2 and RFM 70
RFM of CH2 = 14 70/14 = 5 molecular formula = 5 x CH2 = C5H10
how is water crystalisation written into a formula
CuSO4(salt) . 5H2O (water of crystalisation)
what is the mole ratio always in the form of
1 : x
what is water of crystalisation calculated from
mases of salt and water produced on heating
how would you calculate ‘x’ if 16.7g of MgCl2 . xH2O crystals decreased in mass to 9.5g on heating
1- calculate the mass of water lost —– 16.7 - 9.5 = 7.2g 2- draw a mass, moles, ratio table
what do the balancing numbers in a balanced equation for a reaction tell us
how many moles of each reactant are needed how many moles of each product are made
what is a limiting substance
if there isn’t enough
what is an excess substance
if there is more than enough
what is the calculation for percentage yield
%yield = experimental(from experiment)/ theoretical (from calculation) x 100
what is the experimental mass measured in
an experiment or given to you in the question
what is the theoretical mass based on
the mass of the reactant
what are the steps to calculating percentage yield
1- find or make a balanced equation for reaction 2- calculate theoretical mass that should have been produced 3- calculate the percentage yield using experimental mass from question