Conversion Factors: Intro to
Stoichiometry
3221.MATH.1
3221.MATH.2
How many minutes have you been
alive?
To figure your age: Take your age and write it down,
followed by a decimal. Figure the number of months
since your last birthday, and divide this by 12. This
number will go after a decimal.
Suppose you spent 2.0 hours working on chemistry
homework last night. How many minutes is this?
• You knew the answer to this problem because you
knew an inequality 1 hr = 60 min
• Two conversion factors can be written for all
inequalities
Numerator
Denominator


60 min
1 hour
and
1 hour
60 min
It is important that the equality you select to construct a
conversion factor is a true relationship!!!
Some Common Equalities
Quantity
U.S. (Standard)
Metric (SI)
Metric-Standard
Length
1 ft = 12 in
1 km = 1000 m
2.54 cm = 1 in
1 yd = 3 ft
1 m = 1000 mm
1 m = 39.4 in
1 mile = 5280 ft
1 cm = 10 mm
1 km =0.621 mi
1 qt = 4 cups
1 L = 1000 mL
946 mL = 1 qt
1 qt = 2 pints
1 dL = 100 mL
1 L = 1.06 qt
1 gal = 4 qts
1 mL = 1 cm3
1 lb = 16 oz
1 kg = 1000 g
1 kg = 2.20 lb
1 g = 1000 mg
454 g = 1lb
Volume
Mass
Time
1 hr = 60 min
1 min = 60 s
Conversion Factors
• When an equality shows a relationship for two
units from the same system, it is considered a
definition and exact.
– It is NOT used to determine significant figures
Write conversion factors for the relationship for the
following pairs of units:
a. milligrams and grams
b. minutes and hours
c. quarts and milliliters
Conversion factors stated within a problem
Many times, an equality is specified within a problem
that is true only for that problem.
1. The motorcycle was traveling at a speed of 85
kilometers per hour.
2. One tablet contains 500 mg of vitamin C.
Write the possible conversion factors for
each of the following statements:
1.
2.
3.
4.
There are 325 mg of aspirin in 1 tablet.
One kilogram of bananas cost $1.25.
Wine is 12% alcohol by volume.
A cyclist in the Tour de France bicycle race rides at
the average speed of 62.2 km/hr.
5. A 10.0 g sample of sterling silver has a volume of
94.8 cm3.
Problem Solving
Problem solving in chemistry often requires the
conversion of an initial quantity given in one unit to
the same quantity but in different units.
Numerator
Given quantity x Conversion factor(s) = Desired quantity
Conversion factor(s)
Denominator
(initial unit)
(final unit)
Guide to Problem Solving Using Conversion
Factors
1. Always start with what you are given!! (This will be
at the top left of your conversion)
2. The units you want to end up with will be at the
right of your problem in the same location
(top/bottom)
3. All units should cancel out except for the one(s)
wanted. Conversion factors should be set up to
ensure cancelling of unwanted units.
4. Numbers across top will be multiplied. The
numbers in the denominator are divided.
How many minutes are in 2 years?
The recommended amount of sodium in the
diet per day is 2400 mg. How many grams of
sodium is that?
1 g = _____ mg
If 1890 mL of orange juice is prepared from
concentrate, how many liters of orange juice is
that?
1 L = _______ mL
An antibiotic dosage of 500 mg is ordered. If the
antibiotic is supplied in liquid form as 250 mg in
5.0 mL, how many mL would be given?
One medium bran muffin contains 4.2 g of fiber.
How many ounces (oz) of fiber are obtained by
eating three medium bran muffins if 1 lb=16 oz.
You need 4.0 ounces of a steroid ointment. If
there are 16 oz in 1 lb, how many grams of
ointment does the pharmacist need to prepare.
A physician has ordered 0.50 mg of atropine,
intramuscularly. If atropine were available as
0.10 mg/mL of solution, how many milliliters
would you need to give?
During surgery, a patient receives 5.0 pints of
plasma. How many milliliters of plasma were
given? (1 quart = 2 pints)
A patient needs 0.024 g of a sulfa drug.
There are 8-mg tablets in stock. How many
tablets should be given?
The physician has ordered 1.0 g of tetracycline to be
given every 6 hours to a patient. If your stock on hand
is 500-mg tablets, how many will you need for 1 day’s
treatment? (Hint: Begin with 1 day)