Percentage Yield
In any chemical reaction reactants are converted to products. This is written as:
reactants
products
A simple example would be burning magnesium in oxygen to form magnesium oxide.
magnesium
+
oxygen
reactants
2Mg
+
2 x 24
= 48g
O2
magnesium
oxide
products
2MgO
2 x 16
=32g
2(24+16)
= 80g
Using atomic masses Mg = 24, O = 16 (from the Periodic Table)
This tells us if that 48 g of magnesium ( powder or lump) reacts with 32 g of oxygen then a
MAXIMUM YIELD OF 80 g of magnesium oxide will be produced.
So 48 g of magnesium (Mg) produces 80 g of magnesium oxide (MgO).
1g of magnesium (Mg) produces 80/48 g = 1.66g of magnesium oxide (MgO)
6g of magnesium (Mg) produces 6 x 1.66 g = 10.0g of magnesium oxide (MgO)
This is the MAXIMUM (100 % )YIELD
Fred heated 6 g of magnesium in oxygen, he found he only got 8.5 g of magnesium oxide.
He didn't do very well as he should have got 10.0 g of magnesium oxide ( this he worked out from
the equation). So:
the mass he got = 8.5 g
His % yield was =
(mass he got x 100) / maximum mass
= (8.5 g x 100) /10
= 85%
His % yield was = 85 % - where did the rest go ?
Other examples.
1. In the reaction between methane (CH4) and oxygen (O2) carbon dioxide and water are produced.
The word equation is:
(Atomic masses C = 12, H = 1, O = 16)
methane
+
oxygen
CH4
+
2O2
carbon
dioxide
CO2
(2 x 16)g
= 32g
12+ 2 x 16
= 44g
(12 + 4)g
= 16g
+
water
+
2H2O
2 x (2 + 16) g
= 36 g
This tells us if that 16 g of methane gas react with 64 g of oxygen then a MAXIMUM (100%)
YIELD OF 44 g of carbon dioxide and 36 g of water will be produced.
a) Calculate the maximum masses of carbon dioxide and water produced when 4 g of methane were
burnt in air.....................................................................................................................................
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b) Fred burnt 4g of methane and found that he got 8.8g of CO2.
Calculate the percentage yield of CO2...........................................................................................
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c) 64 g of methane were burnt in air. How much water and CO2 should have been produced,
assuming 100 % yield....................................................................................................................
amount water = .............................................................................................................................
amount of CO2 = .........................................................................................................................
d) Jill burnt 64 g of methane, she found that she got 168 g of water. Calculate the percentage yield
of water. .........................................................................................................................................
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e) Did Jill do anything wrong - comment on her results
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2. In the reaction between ethane (C2H6) and oxygen (O2) carbon dioxide and water are produced.
ethane
+
oxygen
C2H6
+
3.5 O2
carbon
dioxide
2CO2
+
water
+
3H2O
(Atomic masses C = 12, H = 1, O = 16)
Calculate the reacting masses, i.e the masses of:
ethane reacting
= ............................g
oxygen reacting
= ............................g
carbon dioxide formed
= ............................g
water formed
= ...........................g
This tells us if that ............ g of ethane gas react with .............. g of oxygen then a MAXIMUM
YIELD of ............... g of carbon dioxide and ............. g of water will be produced?
When Glen did this experiment he found that it was only 75% efficient, i.e the % yield of the
products was only 75% of what it should have been. If he started out with 15 g of ethane calculate
how much carbon dioxide and water he would have obtained (at 75% yield).
a) amount of carbon dioxide...........................................................................................................
........................................................................................................................................................
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b) amount of water ........................................................................................................................
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3. When acetylene (ethyne) is burnt in oxygen the oxy - acetylene flame is well over 1000oC sufficient to cut through most metals.
The word equation is:
ethyne
+
oxygen
carbon
dioxide
+
water
C2H2
+
2.5O2
2CO2
+
H2O
(Atomic masses C = 12, H = 1, O = 16)
Calculate the reacting masses, mass of
ethyne reacting
= ............................g
oxygen reacting
= ...........................g
carbon dioxide formed
= ............................g
water formed
= ...........................g
This tells us if that ..............g of ethyne gas react with.........g of oxygen then a MAXIMUM YIELD
of ......g of carbon dioxide and.............g of water will be produced.
If 10 g of ethyne were burnt how much carbon dioxide and water were produced if the yield was
a) 100 %
i) amount of carbon dioxide...........................................................................................................
........................................................................................................................................................
........................................................................................................................................................
ii) amount of water ........................................................................................................................
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b) 60% yield
i) amount of carbon dioxide...........................................................................................................
........................................................................................................................................................
........................................................................................................................................................
ii) amount of water ........................................................................................................................
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c) When Carol burnt 25g of ethyne she obtained 60g of carbon dioxide. Calculate her % yield
........................................................................................................................................................
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d) If ethyne was burnt in a limited amount of oxygen what would happen to the cutting power of the
flame i.e. would the flame temperature go up or down - explain your answer.
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HOMEWORK: In your exercise books do the following questions.
1. Nitrogen and hydrogen react together to form ammonia gas.
The reaction is as follows:
a) What does the
nitrogen
+
hydrogen
N2
+
3H2


ammonia.
2NH3
 sign mean ?
b) Using the atomic masses N= 14, H = 1, calculate the reacting masses of nitrogen, hydrogen and
ammonia.
c) In this experiment the conversion of nitrogen and hydrogen into ammonia is only 20% efficient.
If you start with 112 g of nitrogen how much ammonia can be made (at 20% efficiency) ?
d) In this experiment an iron oxide catalyst is used.
What does a catalyst do to the speed of a chemical reaction ?
2. Ammonia reacts with nitric acid to form the fertiliser ammonium nitrate.
a) Write down the word equation for this reaction.
b) If the chemical formula of nitric acid is HNO3 and ammonium nitrate is NH4NO3 write down the
balanced chemical equation for this reaction.
c) Using the atomic masses N= 14, H = 1, O = 16, calculate the reacting masses of nitric acid,
ammonium nitrate and ammonia.
d) Assuming 100% yield if you started off with 85 g of ammonia how much
(i) nitric acid would you need to totally react with this amount of ammonia ?
(ii) ammonium nitrate would you make?
e) A fertiliser sack contains 50 kg of ammonium nitrate. How much ammonia and nitric acid would
you need in order to make this amount of fertiliser ?