Chemistry Reference
In-Depth Information
Anything divided by itself is equal to 1, so the left side of this equation is 1.
The right side of the equation is thus equal to 1 and therefore may be used to
multiply any quantity to change its
form
without changing its
value.
a
100 cents
1 dollar
2.67 dollars
b
267 cents
In any factor, the numerator is equal or equivalent to the denominator, so the
value of the number multiplied is not changed, even though the units are.
EXAMPLE 2.4
In the United States, troy weight is used to measure gold and silver and avoir-
dupois weight is used for most other things. Given that
1 troy pound
12 troy ounces
1 avoirdupois pound
16 avoirdupois ounces
and that ounces in the two systems are
almost
the same weight, calculate the
number of (a) troy ounces in 2.50 troy pounds. (b) the number of avoirdupois
ounces in 2.50 avoirdupois pounds.
Solution
a
12 troy oz
1 troy lb
2.50 troy lb
b
30.0 troy oz
a
16 avoirdupois oz
1 avoirdupois lb
2.50 avoirdupois lb
b
40.0 avoirdupois oz
Practice Problem 2.4
(a) Which weighs more, a pound of bricks or a
pound of lead. (b) Which weighs more, a pound of gold or a pound of lead.
It may be necessary to use more than one factor to get a desired answer. The
factors may be used in separate steps or may be combined in a single step.
EXAMPLE 2.5
Calculate the number of seconds in 5.175 h.
Definition
of hour
Hours
Minutes
Solution
a
60 minutes
1 hour
5.175 hours
b
310.5 minutes
Definition
of minute
Minutes
Seconds
a
60 seconds
1 minute
310.5 minutes
b
18,630 seconds
