quantative chem Flashcards
how do you calculate relative formula mass?
sum of all the RAMs in a compound
eg MgCl2 = 24 + (35.5 x 2) = 95
how do you calculate percentage formula mass?
- calculate RFM
- calculate mass of the elemnt in the compound
- mass of element/ RFM x 100
how do you calculate atom economy?
% atom economy = (mass desired product/ total mass of reactants) x 100
what is avogadro’s number (1 mole) ?
6.02 x 10^23
what is 1 mole?
a measure of how many particles in a substance eg in 23g of Na = 1 mole of Na atoms
moles = ?/?
mole = mass/ RAM or RFM
how do you calculate empirical formulae?
- mass
- RAM
- moles (by doing mass/RAM)
- look for simplest ratio by dividing through by smallest number
what is the empirical formulae?
silmplest ratio of atoms of each element in a compound
what is the moleular formulae?
the actual number of atoms of each element in a compound or molecule
how do you calculate molecular mass?
- molecular mass/ Rfm = molecular formulae
- multiply the imperical formula to find the molecular formulae
- eg imperical formulae = CH
molecular mass = 78
78/ 12 +1 = 6
molecular formulae = C6H6
how do you calculate molar volume of gases?
no. of moles = volume of gas in dm^3 / 24 dm^3
law of conservation of mass
mass cannot be created or destroyed only changed
how do you calculate the ratio of water of crystallisation?
- work out moles of salt and water (mass/ Mr)
- divide by smallest number to find ratio of salt: water
work out ‘emperical formulae’ of salt:water
how does law of concentration mass apply to percipitation reactions?
mass of reactants is exactly the same as mass of products
why does mass appear to decrease/increase in a non-enclosed system?
- if oxidized, mass will ‘increase’ because oxygen (one of the reactants) was not included in the total mass of reactants, but included in the total mass of products
- if gas is ‘lost’ top the atmosphere i the reaction, it will ot be included in the total mass of products so it will appear to have decreased
MASS HAS NOT BEEN DESTROYED OR CREATED
what does a balanced equation show?
correct proportions of reactants that react and products formed
aka stoichiometric reacting masses
how do you calculate masses of reacting substances in chemical reactions? (4 steps)
- work out ratio from the equation of number of moles eg CaCO2 : CaO = 1:1
- calculate moles of known substance (mass/Mr)
- find moles in other substance using ratio
- caluclate mass of unknown substance from number of moles ( mass = moles x rfm)
why might you not get as much product as you would expect? (4)
- reaction is incomplete or reversible
- side reactions or alternate products made
- impure reactions
- product lost during purficatio or isolatation
how do you calculate percentage yield?
- calculate max amount of product (theoretical yield) using the ratio of the equation to calculate how many moles of the product there are, and then mass = moles x rfm
- actual yield/ theoretical yield x 100 = % yield
what is the atom economy?
an indication of the amount of starting material that is converted into useful products
how do you find out stoichimetric coefficients (balancing numbers) of an equation?
- convert masses to moles using moles=mass/rfm
- divide through by smallest number of moles to find simplest ratio
- make sure they are all integers
- write out and balance the equation
when does an equation stop?
when all of one reactant is used up
what is a limiting reagent?
reactant that is fully used up in the reaction
what is an excess reagent?
the other reactant(s) which are not fully used up
how do you work out which is the limiting vs excess reagent?
- work out the moles of each reagent
- the larger one is excess, smaller is limiting
BUT check if there is a ratio that isnt 1:1
eg 2:1 ratio, 0.05 moles of X to 0.08 moles of Y, divide 0.08/2 = 0.04, so X is in excess whilst Y is the limiting factor
how do you calculate the mass produced when you know the masses of reagents?
multiply moles of limiting reagent by rfm to find mass of product
