Moles - 4th
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what is relative formula mass?
• the mass of a formula unit of a substance
how is RFM calculated?
• by adding the relative atomic masses (RAMs) of the atoms or ions.
calculate the RFM for H20.
RFM = (2*1) + 16 = 18
calculate the RFM for Ba(OH)2.
RFM = 137 + (2*(16+1)) = 171
calculate the RFM for CuSO4 dot 5H20.
RFM = 63.5 + 32 + (416) + (518) = 249.5
what is percentage by mass?
• shows the mass of a substance that comes from a particular element.
how is percentage by mass calculated?
• from the formula for the substance, using the RFM
calculate the percentage of C in C3H8.
%C = (C3/C3H8) * 100 = ((312) / ((312)+8) )* 100 = 81.8%
calculate the percentage of N in NH4NO3.
%N = ((N+N)/NH4NO3) * 100 = ((214) / 80) 100 = 35%
what is a mole?
• the unit of amount of substance in chemistry • it represents how many particles of a substance there are, taking into account that particles of different substances have different masses
how is the amount of substance in moles calculated?
• from its mass and RFM • moles = mass (g) / RFM
calculate the amount in moles in 11g of CO2.
= mass / RFM = 11/44 = 0.25 mol
calculate the mass of 2.5 moles of H2O.
= moles * RFM = 2.5 * 18 = 45g
what does the empirical formula of a substance contain?
• the simplest mole ratio of its elements
how can empirical formula be calculated?
• from the masses (or percentages) of each element 1. convert the masses (or percentages) into moles 2. cancel down the mole ratio 3. write down the simplest mole ratio into a formula
calculate the empirical formula of a substance containing 6.2g P and 4.8g O .
• draw a ‘mass, moles, ratio’ table and stick the numbers in: P O mass (g) 6.2 4.8 moles 0.2 0.3 ratio 2 3 • whole number ratio = 2:3 • P203
calculate the empirical formula of a substance containing 83.7% C and 16.3% H.
• use the percentages as masses (as if we had 100g of the substance • draw a ‘mass, moles, ratio’ table and stick the numbers in: C H mass (g) 83.7 16.3 moles 6.975 16.3 ratio 3 7 • whole number ratio = 3:7 • C3H7
what leads to errors in empirical formula calculations?
• rounding moles is v.dangerous and can lead to errors • you can get away with rounding moles to 2dp, but never round to 1dp or whole numbers
what does the molecular formula of a substance contain?
• the actual numbers of atoms in a molecule - like empirical formula, but uncancelled
how can the molecular formula be calculated?
• from the empirical formula and RFM • this works by finding the uncancelled multiple of the empirical formula with the correct RFM
calculate the molecular formula if the empirical formula is CH2 and the RFM is 70.
• RFM of CH2 = 14 • multiple = 70/14 = 5 • molecular formula = 5 * CH2 = C5H10
what do hydrated salt crystals contain?
• water trapped inside them in a fixed mole ratio • this water is called water of crystallisation and it is written into a formula like so: CuSO4 “DOT” 5H2O salt “dot” water of crystallisation
what happens to hydrated salt crystals when they are heated?
they lose their water as vapour - e.g. CuSO4”dot”5H2O -> CuSO4 + 5H2O
how is water of crystallisation calculated?
• from the masses of salt and water produced on heating