Many problems in chemistry require a change in units.

Conversion Factor
A ratio in which the numerator and denominator are quantities from an equality or given relationship.

Example:

equality		2 conversion factors
	as		or
1 kg = 2.20 lbs		2.20 lb  1 kg		1 kg  2.20 lb

• When written properly, conversion factors for an equality are inverses of one another.

Sometimes, an equality is specified within a problem that is true only for that problem.

Examples:

• Write an equality and two conversion factors for the following:

2.  One tablet contains 500 mg of vitamin C.
 

Equality:	2 Conversion Factors
		and
1 tablet = 500 mg	500 mg  1 tablet		1 tablet  500 mg

3.  Gold was priced at $1,187 an ounce.
 

Equality:	2 Conversion Factors
		and
1 oz gold = $1,881	$1,881  1 oz gold		1 oz gold  $1,881

How can two conversion factors be written for one equality?
 

Practice Problems Answer the following questions:

1. Write an equality and two conversion factors for the for the following: . a)  There are 325 mg of aspirin in 1 tablet . b)  One kilogram of bananas costs $1.25.

Conversion Factors for Percentages

Sometime the ratio for contents of a mixture will be expressed as percentages of the whole.

Percentages
The term percent (%) means “per one hundred.”

Example:

Percent:	18% body fat by mass
Equality:	18 kg body fat = 100 kg body mass
Conversion factors:	18 kg body fat  and   100 kg body mass  100 kg body mass 18 kg body fat

What 'percent' mean?
 

Practice Problems Answer the following questions:

1. Write an equality and two conversion factors for the following: . The wine was 15% by volume:

Conversion Factors for ppm & ppb

To indicate ratios with very small percentages, parts per million (ppm) or parts per billion (ppb).

Parts Per Million (ppm)
The ratio parts per million indicates the milligrams of a substance per kilogram (mg/kg).

Parts Per Billion (ppb)
The ratio parts per million indicates the micrograms of a substance per kilogram (µg/kg).

Example: Parts Per Million:

The maximum amount of lead allowed by the FDA in pottery glaze on small bowls is 5 ppm, or 5 mg/kg.
 

ppm:	5 ppm lead in pottery glaze
Equality:	5 mg lead = 1 kg glaze
Conversion factors:	5 mg lead   and    1 kg glaze   1 kg glaze 5 mg lead

What are the units for ppm and ppb?
 

Practice Problems Answer the following questions:

1. Write an equality and two conversion factors for the following: . The EPA has set the maximum level for mercury (Hg) in tuna at 0.1 ppm.