Empirical and Molecular Formulae

AS Worksheet 1.2(6) Based on Chemsheets by RW Grime (www.chemsheets.co.uk)

CALCULATIONS ON PERCENTAGE YIELD & ATOM ECONOMY

Answer the following questions on file paper. Remember to set out your working neatly and clearly, showing all necessary steps. Give answers to the appropriate number of significant figures. Use the Ar values for each element as given on your copy of the Periodic Table.

1) In the Contact Process, sulfur dioxide reacts with oxygen to make sulfur trioxide:-

2SO2 + O2  2SO3

a) Calculate the maximum theoretical mass of SO3 that can be made by reacting 96g of SO2 with an excess of oxygen.

b) Under certain reaction conditions, only 90g of SO3 were made from 96g of SO2. Calculate the percentage yield. [3]

2) Iron is extracted from iron (III) oxide in the Blast Furnace by reduction with carbon monoxide as shown below:-

Fe2O3 + 3CO  2Fe + 3CO2

a) Calculate the maximum theoretical mass of iron that can be made from 1 tonne of iron (III) oxide.

b) Calculate the percentage yield if only 650kg of iron was obtained. [3]

3) Titanium is manufactured by reacting titanium (IV) chloride with magnesium:-

TiCl4 + 2Mg  Ti + 2MgCl2 Calculate the percentage yield in the reaction where 20 tonnes of titanium are obtained from 100 tonnes of titanium (IV) chloride. [2]

4) Aluminium is extracted from purified aluminium oxide by electrolysis:-

2Al2O3  4Al + 3O2 Calculate the percentage yield for the case where 500g of aluminium are obtained from 1kg of aluminium oxide. [2]

5) Ammonia is manufactured by the Haber Process by reacting hydrogen and nitrogen:-

N2 + 3H2  2NH3 Calculate the percentage yield for the case where 153 tonnes of ammonia are obtained from the reaction of 90 tonnes of hydrogen with excess nitrogen. [2]

6) Ammonia is used to make the fertiliser ammonium sulfate by reaction with sulfuric acid:-

2NH3 + H2SO4  (NH4)2SO4 In a lab experiment, 85g of ammonia were reacted with excess sulfuric acid and 300g of ammonium sulfate was obtained. Calculate the percentage yield. [2]

1 AS Worksheet 1.2(6) Based on Chemsheets by RW Grime (www.chemsheets.co.uk)

7) Calculate the percentage atom economy of the named product in the reactions shown below:- [12]

a) sodium in the reaction 2NaCl  2Na + Cl2

b) hydrogen in the reaction Zn + 2HCl  ZnCl2 + H2

c) iron in the reaction Fe2O3 + 3CO  2Fe + 3CO2

d) calcium oxide in the reaction CaCO3  CaO + CO2

e) sulfur trioxide in the reaction 2SO2 + O2  2SO3

f) oxygen in the reaction 2H2O2  2H2O + O2

8) Hydrazine, N2H4, was used as rocket fuel for the Apollo missions to the Moon. It can be made by the reaction of ammonia with sodium chlorate (I), NaClO. The reaction is shown below:-

2NH3 + NaClO  N2H4 + NaCl + H2O

a) Calculate the maximum theoretical mass of hydrazine that can be made by reacting 340g of ammonia with excess sodium chlorate (I).

b) Calculate the percentage yield if only 280g of hydrazine were obtained.

c) Calculate the atom economy for this method of producing hydrazine.

“ HOW SCIENCE WORKS” QUESTION 9) Ibuprofen is a common anti-inflammatory drug often used as a pain killer for headaches, toothache and muscular pains, amongst other things. It was developed by Boots in the 1960s and was initially marketed as Brufen to treat rheumatoid arthritis. It became available without prescription in the 1980s and is sold under different tradenames. The atom economy in the original method for making ibuprofen was 40%. A new manufacturing process invented in the 1980s had an improved atom economy of 77%. This is also a cheaper process. Sustainable development aims to enable all people throughout the world to satisfy their basic needs and enjoy a better quality of life without compromising the quality of life of future generations. Generally, the higher the atom economy in a process, the better it is for sustainable development because there is less waste,

a) The newer method of manufacture has almost double the atom economy of the original process. Explain why a higher atom economy is better.

b) About 3000 tonnes of ibuprofen tablets are used in the UK each year. Calculate the mass of waste produced by making 3000 tonnes of ibuprofen by the original method.

c) How much less waste is produced making 3000 tonnes of ibuprofen by the newer method used nowadays compared to the old method? [5]