maths Flashcards
what to do if the abundance is not a percentage in relative atomic mass
times the abundance numbers as normal with mass but then instead of dividing by 100 divide by the total of the abundance numbers.
what to do if you want to find the percentage abundance of an isotope but you have two unknown percentages
set one of them as equal to x and then because a percentage is always out of 100 you should set the other as 100-x
then use them to form an equation and work out x as you only have one variable (x)
when doing mass/ mr mole calculations and we are trying to find the mr (RFM) in the calculation for the compound do we take into account the big numbers
no we only use them for the mole ratio
what is the empirical formula and how to work it out from no.of moles
smallest whole number mole ratio of substances in a formula
work out NO. of moles of each substance by mass over MR (RFM)
this is the mole ratio
divide by the smallest NO. of moles to make this substance have a mole ratio of 1
after dividing each NO. of moles by the smallest NO. of moles this is your empirical formula
what do we do when we get given percentages instead of masses when trying to work out NO. of moles using MASS/MR MOLE
just treat them as masses
do the percentage divided by RFM for the NO. of moles
how to determine the molecular formula from the empirical formula and the molecular mass
because the empirical formula is just the most simplified ratio
the molecular formula is a just a multiple of the empirical formula
so just work out the formula mass of the empirical formula
divide the mass you are given in the question with the formula mass you just worked out
you have just worked out the scaler from the empirical formula to the actual molecular formula
so times the empirical formula by the scaler you just worked out to get the actual molecular formula
steps of how to find moles of substances
- write the mass or percentage of each compound
- write the RFM of each compound
then just divide the mass of RFM (MR) to get the moles
when would we use molecular vs empirical formula
molecular usually in simple covalent where there is a small amount of atoms we can count eg 2 carbon 6 hydrogen and 1 oxygen = C2H6O
empirical- giant substances eg giant covalent as the NO. of atoms is extremely large and it is just not practical to write the actual amount of them in the formula.
so empirical formula is used to show the simplest whole number ratio of each atoms in a compound
when asked to calculate the amount of a substance or compound
amount just means moles
so we just do mass/ MR
how to work out the number of atoms of a compound
find out the (amount) number of moles of the compound
see how many atoms make up that compound
eg NH3= 4 atoms
Divide the overall number of moles of the compound by the number of atoms in the compound to get overall number of atoms
when using avogadros constant to work out amount remeber
if it is a compound and you work out the amount of the compound to work out the amount of atoms
you would just divide the amount (no. of moles) of the whole compound by how many atoms are in that compound eg NH3= 4 atoms
pretty intuative
to work out number of electrons you just have to find amount of atoms and times that by how many electrons are in that atom which you find just from periodic table
Mass/mr mole explained
Moles is how much there is (amount)
So the mass is moles times by MR
As we want to know how much the amount weighs
The moles is mass/ Mr
As the mass is the mass of the whole formula Which obviously is the amount of molecules(no of moles) times by their individual weight (Mr)
