Chemistry Reference
In-Depth Information
Practice Problem 10.15
Calculate the mass of nitrogen in the ammo-
nium phosphate,
10
6
g
(NH
4
)
3
PO
4
,
prepared by treating
6.15
of phosphoric
acid with excess aqueous ammonia.
Snapshot Review
❒
The conversions of Sections 10.1 and 10.2 are extended in this
section, using Avogadro's number, density, and chemical formulas
(moles of one element per mole of any other substance).
A. Calculate the mass of
PCl
3
required to prepare the quantity of HCl that
10
23
contains
4.55
chlorine atoms by the following reaction:
PCl
3
(
/
)
3 H
2
O(
/
) £ H
3
PO
3
(
/
)
3 HCl(g)
10.4
Problems Involving Limiting Quantities
In problems in the preceding sections, a quantity of one reactant was given, and
it was assumed that enough of any other reactants was present. In Example 10.3,
for instance, 0.750 mol of hydrochloric acid reacted with calcium. If no calcium
is present, however, no reaction is possible—no matter how many moles of
hydrochloric acid there are. In Example 10.3, we assumed that sufficient cal-
cium was present because nothing was stated about the quantity of calcium used.
In contrast, problems involving limiting quantities have the quantities of at least
two reactants given. The reactant used up first limits the quantities of the prod-
ucts and is referred to as being present in
limiting quantity.
Any other reactant
may be present in an amount that represents a number of moles equal to or
greater than is required for the reaction and is said to be present in sufficient or
excess
quantity. The reaction is said to have gone to
completion
when the lim-
iting quantity has been used up. An example from everyday life illustrates this
principle.
The reactant that is present in
limiting quantity determines the
extent of reaction that can take
place.
EXAMPLE 10.16
(a)
If cashew nuts cost $8.00 per pound, how many pounds of these nuts can
be purchased with $108.00?
(b)
How many pounds of cashew nuts can be purchased with $108.00 if the
store has 12.5 lb in stock?
Solution
(a)
Assuming that the store has sufficient nuts,
Price
per pound
Dollars
Pounds
1 lb
8.00 dollars
108.00 dollars
a
b
13.5 lb
