IB Chemistry Power Points
Topic 1
Quantitative Chemistry
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Lecture 3
Limiting Reactant
Percent Yield
Consider the following reaction
2 H2 + O2  2 H2O
Reactants are combined in
perfect proportions
3 molecules
6 molecules
6 molecules
In reality this never happens
3 molecules
6 molecules
6 molecules
Consider
4 molecules
3 molecules
4 molecules +
leftover oxygen
Consider
LIMITING
REACTANT
EXCESS
REACTANT
Amount of
PRODUCT is
determined
by limiting
reactant
Consider
6 molecules
2 molecules
4 molecules +
leftover
hydrogen
Consider
EXCESS
REACTANT
LIMITING
REACTANT
Amount of
PRODUCT is
determined
by limiting
reactant
Given 24 grams of O2 and 5.0 grams of H2
determine the mass of H2O produced.
2 H2 + O2  2 H2O
the mass of H2O produced will be determined by the
limiting reactant - do TWO calculations
calculation for 24 grams of O2
24 g O2
32.0 g mol-1
2 H2O
1 O2
18.0 g mol-1
= 27 g of H2O
calculation for 24 grams of O2
24 g O2
32.0 g mol-1
2 H2O
1 O2
18.0 g mol-1
= 27 g of H2O
calculation for 5.0 grams of H2
5 g H2
2.0 g mol-1
2 H2O
2 H2
18.0 g mol-1
= 45 g of H2O
calculation for 24 grams of O2
24 g O2
32.0 g mol-1
2 H2O
1 O2
18.0 g mol-1
= 27 g of H2O
O2 is the LIMITING REACTANT and determines the
amount of product
calculation for 5.0 grams of H2
5 g H2
2.0 g mol-1
2 H2O
2 H2
18.0 g mol-1
= 45 g of H2O
H2 is the EXCESS REACTANT (some would be left
over)
How much hydrogen gas would be left over?
To calculate, first determine how much reacts
with all of the oxygen
How much hydrogen gas would be left over?
To calculate, first determine how much reacts
with all of the oxygen
given 24 grams of O2
24 g O2
32.0 g mol-1
2 H2
1 O2
3.0 g of H2 reacts so
2.0 g mol-1
= 3.0 g of H2
How much hydrogen gas would be left over?
To calculate, first determine how much reacts
with all of the oxygen
given 24 grams of O2
24 g O2
32.0 g mol-1
2 H2
1 O2
2.0 g mol-1
= 3.0 g of H2
3.0 g of H2 reacts so
5.0 g – 3.0 g = 2.0 g of hydrogen remains
Percent Yield
Enoch the Red, an alchemist,
wants to try to turn lead into
gold (which you can’t do
chemically). He finds that
mixing lead with an
unidentified compound (gold
III chloride) actually produces
small amounts of gold. The
reaction is as follows:
2 AuCl3 +3 Pb  3 PbCl2 + 2 Au
Percent Yield
Enoch reacts 14.0 g of gold III
chloride with excess lead
metal. What would be the
maximum, THEORETICAL yield
of this reaction?
2 AuCl3 +3 Pb  3 PbCl2 + 2 Au
Percent Yield
Enoch reacts 14.0 g of gold III
chloride with excess lead
metal. What would be the
maximum, THEORETICAL yield
of this reaction?
2 AuCl3 +3 Pb  3 PbCl2 + 2 Au
Percent Yield
Enoch reacts 14.0 g of gold III
chloride with excess lead
metal. What would be the
maximum, THEORETICAL yield
of this reaction?
2 AuCl3 +3 Pb  3 PbCl2 + 2 Au
given 14.0 g of AuCl3
14.0 g AuCl3
303.5 g mol-1
2 AuCl3
2 Au
196.97 g mol-1
= 9.09 g Au
Percent Yield
Enoch recovers only 1.05 g of
gold from the reaction. This
could be for many different
reasons
some product was lost in the recovery process
the reaction did not go to completion
the AuCl3 is not pure
Percent Yield
the percentage yield expresses
the proportion of the expected
product that was actually
obtained.
actual
% yield=
×100%
theoretical
1.05
% yield=
×100%=11.6%
9.09