Moles - 4th Flashcards
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
calculate ‘x’ if 16.7g of MgCl2”dot”xH20 crystals decreased in mass to 9.5 on heating.
- calculate the mass of water lost - 16.7g - 9.5g = 7.2g
- draw a ‘mass, moles, ratio’ table and stick the numbers in. be careful to use the final mass of solid for the salt, not the original mass of crystals
MgCl2 H2O
mass (g) 9.5 7.2
moles 0.1 0.4
ratio 1 4
- whole number ratio = 2:3
- x = 4
- MgCl2 “dot” 4H2O
what term do chemists use when there isn’t enough substance to do a reaction?
- the substance is limiting
what term do chemists use when there is more than enough substance to do a reaction?
- the substance is in excess
what happens when we mix 0.9 moles of Zn with 1.1 moles of CuO?
- Zn + CuO -> ZnO + Cu
- the equation tells us the moles of both should be the same, so it must be that:
Zn is limiting, because there isn’t enough of it and CuO is in excess, because there’s too much of it.
what does the limiting substance determine?
- how many moles are actually used and made
how can you work out which substance is in limiting/excess?
- because there are multiple substances in a reaction, each with a mass and potentially different balancing numbers in the reaction equation, it can be difficult to work out which is in excess
- pick one at random, then calculate how much of the other would be needed to react with it, and then compare that to how much of the other there actually is.
what happens if we mix 10g of Mg and 10g of HCl?
- Mg + 2HCl -> MgCl2 + H2
- start with Mg and calculate how much HCl would be needed to react with that amount of Mg, and then compare that to how much HCl there actually is.
- 1Mg : 2HCl
- 0.416 moles : 0.833…
- mass of HCl - 30.4g
- 10g of Mg would need to react with 30.4g of HCl, but we only have 10g of HCl!
- HCl is limiting - there isn’t enough - Mg is in excess
how is percentage yield of a reaction calculated?
% yield = (experimental (from experiment) / theoretical (from calc)) * 100
how is experimental mass and theoretical mass found?
- EM - measured in an experiment (or given to you in question)
- TM - calculated based on the mass of a reactant
calculate the percentage yield if 10g of Al produces 0.96g of H2 in an experiment.
- find/invent a balanced equation for the reaction :
2Al + 6HCl -> 2AlCl3 + 3H2 - calculate the theoretical mass of H2 that 10g of Al should have produced
- 2Al : 3H2
mole ratio - 2 : 3
actual moles - 0.37 : 0.55
- mass of H2 = 1.11g
- % yield = (0.96/1.11) * 100 = 86.4%
