Revision notes for AQA AS Chemistry Unit 2- Amount of Substance

Unit two revision notes for january 2012 exams on AQA syllabus

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Amounts of Substance
Key formulas
Mass= Relative Molecular mass x Moles
Pressure x Temperature=number of moles x Gas Constant x Volume
Percentage yield= Actual yield/Theoretical yield x 100
% Atom economy= Mass of desired product/Total mass of reactants x 100
How to convert between dm3, cm3 and m3

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Doing a titration
First wash out a conical flask with distilled water and a pipette with the alkali that you are
using. This prevents cross contamination.
Then measure out some alkali using a pipette and put it in a conical flask along with some
indicator. Either Methyl Orange or Phenolphthalein
Wash the sides off the flask down with distilled water to check that all the alkali is in it.…read more

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Question based on titrations and using formulas
MgCO3+2HCl MgCl2 +H2O +CO2
When 75.0 cm3 of 0.500 moldm-3hydrochloric acid were added to 1.25g of impure MgCO3 some
acid was left unreacted. This unreacted acid required 21.6cm3 of a 0.500 moldm-3 solution of
sodium hydroxide for a complete reaction.
(i)Calculate the number of moles of HCl in 75.0cm3 of 0.500 moldm-3 hydrochloric acid.
Conc= Moles/Voles
Moles=Concentration x Volume
? = 0.500 moldm-3x 75/1000
0.…read more

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Mass= Mr Moles
Moles= Mass/Mr
1.00g/101.1 =? = 9.89x10-3
Question using Ideal Gas Equation
(ii)At 298 K and 100kPa the oxygen gas produced in this decomposition occupied a volume of
1.22 x 10-4 m3.
State the ideal gas equation and use it to calculate the number of moles of oxygen produced
in the decomposition.
100kPa x (1.22x10-4 m3x1000)/8.31 J K-1mol-1 x 298K= n
= 4.37x10-4
How to work out empirical formula
1.…read more

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1K 2K 1K 2K
1N 2N 1N 2N
3O 6O 4O 6O
2KNO3 2KNO2 + O2
Question on atom economy and % yield.
Cr2O3 + 2Al ---> 2Cr + Al2O3
(i)Calculate % atom economy for Chromium
Mass of desired product/Total mass of reactants x 100
2(52)/(52+52)+(16+16+16)+2(27) =0.5048543689
The electrolysis of water forms H2 and O2.
2H2O 2H2 + O2
(i) What is the % yield of O2 if 12.3 g of O2 is produced from the decomposition of 14.…read more


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