Chemistry Reference
In-Depth Information
Solution
2 H
2
(g)
O
2
(g) £ 2 H
2
O(
/
)
Because the equation gives the mole ratio, the number of molecules is changed
to moles of
O
2
,
then to moles of
H
2
O,
which can then be converted to grams
(mass) of H
2
O:
Balanced
chemical
equation
Molecules
of O
2
Avogadro's
number
Moles
of O
2
Moles
of H
2
O
Molar
mass
Mass
of H
2
O
1 mol O
2
a
2 mol H
2
O
1 mol O
2
a
18.0 g H
2
O
1 mol H
2
O
10
22
O
2
molecules
4.51
a
b
b
b
10
23
O
2
molecules
6.02
2.70 g H
2
O
Practice Problem 10.14
Calculate the number of individual atoms
of barium metal that can react when heated with aluminum oxide to form solid
barium oxide and 7.33 g of aluminum.
The number of moles of an element in a mole of compound can also be
used to calculate the number of moles of the compound involved in a reaction.
The ratio of the number of moles of an element within a compound to the num-
ber of moles of the compound is determined by the compound's chemical for-
mula (Section 7.3). Thus, the subscripts of the formula may be used to form
conversion factors.
EXAMPLE 10.15
The quantities of nitrogen, phosphorus, and potassium in a fertilizer are critical
to the fertilizer's function in helping crops grow. Calculate the number of moles
of nitrogen atoms in the ammonium phosphate, produced by the
reaction of excess aqueous ammonia with 227 mol of phosphoric acid.
(NH
4
)
3
PO
4
,
Solution
The reaction is
3 NH
3
(aq)
H
3
PO
4
(aq) £ (NH
4
)
3
PO
4
(aq)
Balanced
chemical
equation
Moles of
H
3
PO
4
Moles of
(NH
4
)
3
PO
4
Moles of
N atoms
Formula
a
1 mol (NH
4
)
3
PO
4
1 mol H
3
PO
4
3 mol N
1 mol (NH
4
)
3
PO
4
227 mol H
3
PO
4
b
a
b
681 mol N
From the
chemical formula
