Using Chemical Formulas - Madison Public Schools

Chemical Formulas and
Chemical Compounds
Using Chemical Formulas
Formula Mass: sum of the atomic masses
of all the atoms in a molecule or formula
unit; measured in amu
Example: H2O
2  1 . 00794 amu
 15 . 9994 amu
18 . 0153 amu
For water this can also be called the
Molecular Mass
Molar Mass – the mass of one mole of a
substance; numerically equal to the
formula mass; measure in g/mol
• The formula mass of H2O is 18.0153 amu
• The molar mass of H2O is 18.0153 g/mol
Sample Problems
1. What is the mass in grams of 3.04 moles of
ammonia vapor, NH3?
2. Calculate the mass of 0.257 moles of calcium
3. The odor of bananas is due to the compound
isopentyl acetate, C7H14O2. How many moles
of isopentyl acetate are present in a 5.0 gram
sample of the compound?
4. When bees sting, they also release the
compound isopentyl acetate. How many
molecules of the compound are there in 2.00
Sample Problems
5. Dioxin, C12H4Cl4O2, is an extremely toxic
substance. How many moles of dioxin are
there in 700.0 grams of the substance?
6. One type of antibiotic, penicillin F, has the
formula C14H20N2SO4. How many molecules of
penicillin are there in a 250.0 mg dose.
Class Warmup - Part 1
Consider a sample of 10.0 g of the gaseous
hydrocarbon C3H4 . How many molecules are
present in the sample?
Class Warmup - Part 2
1. Express each of the following as a decimal #
a. 56 %
b. 0.592 %
2. Express each of the following as a percentage.
a. 0.10
b. 3 cm3/4 cm3
3. If you lost 25 marbles and found only 10, what
percentage of the marbles did you find? What
percentage is still lost?
4. A 59 g mixture contains 23 g of glucose. What is the
percentage of glucose in the mixture?
Percentage Compositon
Percentage Composition – the percentage
by mass of each element in a compound
mass of element in 1 mol of compound
 100 %
molar mass of compound
Remember: According to the law of
constant composition the percentage
composition for a compound is always
exactly the same.
Percentage Composition
Example: TNT - 2, 4, 6 trinitrotoluene
C – 37.017%
H – 2.21883 %
N – 18.5002 %
O – 42.2644 %
Sample Problems
1. Calculate the percentage composition of
sodium nitrate.
2. What is the mass percentage of water in the
hydrate, CuSO4.5H2O?
3. Zinc chloride, ZnCl2 is 52.02 % chlorine
a. What mass of chlorine is contained in 80.3 g
of ZnCl2?
b. How many moles of Cl is this?

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