topic 3-quantitative chemistry
relative formula mass
percentage mass of an element in a compound= Ar x number of atoms of that element/ Mr of compound x 100
example:
find % mass of sodium in sodium carbonate, Na2CO3...
Ar of Na= 23
Ar of carbon= 12
Ar of oxygen= 16
Mr of Na2CO3= (2x23)+12+(3x16)=106
23x2/106x100= 43%
the mole
number of moles= mass in g/ Mr
example:
how many moles are there in 66g of CO2?
Mr of CO2= 12+(16x2)=44
66/44= 1.5 mol
conservation of mass
during a chemical reaction no atoms are destroyed and no atoms are created
the are the same number of atoms and types of atoms on each side of the reaction equation
no mass is gained or lost- is conserved
if mass seems to change there's a gas produced
the mole and equations
example:
8.1g of ZnO reacts completely with 0.6g of carbon to form 2.2g of CO2 and 6.5g of Zn...
Ar of C=12, Ar of 0=16, Ar of Zn=65
1)ZnO= 65+16=81
CO2=12+(2x16)=44
C=12
Zn=65
2) ZnO=8.1/81=0.1 mol
CO2=2.2/44=0.05
C=0.6/12=0.05
Zn=6.5/65=0.1
3)0.1/0.05=2
0.05/0.05=1
0.05/0,05=1
0.1/0.05=2
4) 2ZnO+C---->CO2+2Zn
limiting reactants
reaction stops when 1 of the is used up. any other reactants are in excess
the amount of product formed is directly proportional to the amount of limiting reactant
example
calculate the mass of Al202 when 135g of Al is burned in air...
4Al+3O2---->2Al2O3
Al=27
Al2O3=102
moles=mass/mr=135/27=5
4:2
5:2.5
mass=moles x mr=2.5x102=225g
gases and solutions
volume of gas=mass of gas/mr of gas x24
concentration= mass of solute (g)/volume of solvent (dm3)
concentration= number of moles of solute (mol)/ volume of solvent (dm3)
concentration calculations
example:
whats the concentration in g/dm3 of sulfuric acid solution...
mr(H2SO4)=98
mass=moles x relative formula mass=0.04166 x 98=4.08g/dm3
atom economy
atom economy= relative formula mass of desired products/ relative formula mass of all reactants x100
example:
calculate the atom economy of...
CH4+H2---->CO+3H2
Mr(CH4)=16
Mr(H20)=18
16+18=34
3 x Mr(H2)=6
6/34 x 100=17.6%
high atom economy is better for profit and the environment
the less products, higher atom economy - no waste products
percentage yield
percentage yield= mass of actual product/ maximum theoretical yield x100
always less than 100%
high py is good for industry- less waste, more profit
lose some product when you separate it from reaction mixture
side reactions can decrease py