174
Stolchlometry II: Calculations Based on Chemical Equations
PROBLEM:
Oxygen is prepared by heating KClO 3 .
(a) What weight of O 2 is obtained from 3.00 g KC1O 3 ?
(b) What is the volume of O 2 , measured at standard conditions?
(c) What volume does the O 2 occupy if collected over water at 730 torr and 25°C?
SOLUTION:
The first step is to write the balanced equation for the reaction (if it is not given).
This step requires knowledge of the experimental facts. We note in the text that,
when KClO 3 is decomposed by heating, the products are KCl and O 2 , so we start
with the unbalanced equation
KClO 3 4> KCl + O 2
(The A symbol indicates that heating is necessary.) To account for the three
moles of oxygen atoms in KC1O 3 we need f moles of O 2 in the products:
KClO 3 4. KCl + fO 2
This is now a balanced equation, but we prefer to eliminate fractional numbers of
moles, so we multiply all terms by 2, getting the final equation
2KC1O 3 4 2KC1 + 3O 2
We now examine the problem, asking two questions: (a) what is given, and (b)
what is sought? We see (a) that the weight of KC1O 3 used is given, and (b) that we
seek the amount of O 2 produced. We then proceed with the three steps listed
earlier.
1. From the weight of KC1O 3 given, compute the number of moles that are
given.
Moles of KC1O 3 given =
3 -°° 8 KC1O 3 = Q Q2 ^ ^^ ^^
122.6
SKC1 °
3
mole KC1O 3
2. From the moles of KC1O 3 given, compute the number of moles of O 2 produced. The chemical equation shows that 3 moles of O 2 are produced from 2
moles of KC1O 3 . Therefore,
Moles of 0 2 produced = ( 2 mo^KCloJ
(0
'
0245 moles KC1
°3)
= 0.0368 moles O 2
3. From the moles of O 2 produced, express the amount of O 2 in the units
specified in the statement of the problem, as follows.
(a) Weight of O 2 = (0.0368 moles O 2 )( 32.0 ^
2
Q j
= 1.18g0 2
(b) The volume of O 2 will be given by the ideal gas law (see p 159). For standard
conditions, T = 273.2 K and P = 760.0 torr, so
Stolchlometry II: Calculations Based on Chemical Equations
PROBLEM:
Oxygen is prepared by heating KClO 3 .
(a) What weight of O 2 is obtained from 3.00 g KC1O 3 ?
(b) What is the volume of O 2 , measured at standard conditions?
(c) What volume does the O 2 occupy if collected over water at 730 torr and 25°C?
SOLUTION:
The first step is to write the balanced equation for the reaction (if it is not given).
This step requires knowledge of the experimental facts. We note in the text that,
when KClO 3 is decomposed by heating, the products are KCl and O 2 , so we start
with the unbalanced equation
KClO 3 4> KCl + O 2
(The A symbol indicates that heating is necessary.) To account for the three
moles of oxygen atoms in KC1O 3 we need f moles of O 2 in the products:
KClO 3 4. KCl + fO 2
This is now a balanced equation, but we prefer to eliminate fractional numbers of
moles, so we multiply all terms by 2, getting the final equation
2KC1O 3 4 2KC1 + 3O 2
We now examine the problem, asking two questions: (a) what is given, and (b)
what is sought? We see (a) that the weight of KC1O 3 used is given, and (b) that we
seek the amount of O 2 produced. We then proceed with the three steps listed
earlier.
1. From the weight of KC1O 3 given, compute the number of moles that are
given.
Moles of KC1O 3 given =
3 -°° 8 KC1O 3 = Q Q2 ^ ^^ ^^
122.6
SKC1 °
3
mole KC1O 3
2. From the moles of KC1O 3 given, compute the number of moles of O 2 produced. The chemical equation shows that 3 moles of O 2 are produced from 2
moles of KC1O 3 . Therefore,
Moles of 0 2 produced = ( 2 mo^KCloJ
(0
'
0245 moles KC1
°3)
= 0.0368 moles O 2
3. From the moles of O 2 produced, express the amount of O 2 in the units
specified in the statement of the problem, as follows.
(a) Weight of O 2 = (0.0368 moles O 2 )( 32.0 ^
2
Q j
= 1.18g0 2
(b) The volume of O 2 will be given by the ideal gas law (see p 159). For standard
conditions, T = 273.2 K and P = 760.0 torr, so