Calculations Flashcards
what does it mean if a reactant is in ‘excess’
some left over/ extra after the reaction has occured
why is it called a ‘limiting’ reagent
limits how much product can be formed
what is the mass of 1 mole
gfm
steps in excess calculations
- find no of moles for each reactant
- write down molar ratio
- sub in actual no of moles of reactant 1 and see how much reactant 2 is needed
- if have enough then r2 is in excess and r1 is limiting reagent, if not sub in other way around
steps on how to find mass of product formed
- work out which reactant is in excess and which is limiting reagent
- work out no of moles of product will be produced based on moles of LIMITING REAGENT
- use mass,n,gfm triangle to calculate mass of product formed
purpose of percentage yield calculations
chemical reactions do not produce 100% of the product expected and the actual yield is less than this.
percentage yield equation
%yield=actual yield/theoretical yield x100
what is generally considered to be a good %yield?
above 70%
steps for percentage yield calculation?
- write out molar ratio
- work out number of moles of reactant
- work out number of moles of product
- work out mass of product (theoretical yield)
- find %yield using theoretical and actual yield
what is molar volume?
the volume occupied by one mole of gas
units of molar volume
litres/mol
the molar volume is the same…
for all gases at the same temperature and pressure
approximate molar volume of gases at 20 degrees is…
24 litres/mol
approximate molar volume of gases at 0 degrees is…
22.4 litres/mol
molar volume equation
molar volume = volume/no. of moles
determining molar volume experimentally
- work out mass of gas (mass of flask+gas-mass of flask)
- work out no. of moles n=m/gfm
- sub in no. of moles to molar volume equation
way to work out mass of reactant mixture without calculator
molar ratio
reacting volumes (gas in a reaction) steps
- calculate no of moles of substance in the question
- use molar ratio
- use molar volume relationship
excess and molar volume steps
- find the limiting reagent
- use molar ratio to find moles of what you are looking for
- use molar volume relationship
2 ways the efficiency of reactants being converted into products can be measured:
- percentage yield
2. atom economy
%yield: after a chemist makes a new product…
they allow the product to ‘dry’ so that any water/solvent is removed.
watch glass weighed again with new substance on - difference in masses = actual mass of product
can then carry out %yield calculation
percentage yield provides a measure of…
the degree in which the limiting reagent is converted to the desired product
what does percentage yield not take into account
by-products formed that could be problematic and wasteful
atom economy measures the…
proportion of the total mass of reactants converted into the desired product.
reactions which have a high percentage yield may have a low atom economy if…
large quantities of unwanted by-products are formed
atom economy equation
%atom economy= mass of desired products / total mass of reactant x100
100% atom economy means that…
all reactants are turned into products. This is desirable
Situation where there is 100% atom economy
if there is only 1 product
steps for calculating atom economy
- work out mass of desired product (no. of moles x gfm)
- work out total mass of reactants
- atom economy equation
clean by-products that are easy to dispose of
water
comparing two processes
work out atom economy for each and compare by-products - are they wasteful, toxic, flammable etc?
steps for redox titration calculations:
- work out no. of moles of substance of known concentration
- use molar ratio to work out no. of moles of substance of unknown concentration
- calculate concentration using c=n/v
