Chemical Formulas and Chemical Compounds

Using Chemical Formulas

Definitions:

Formula Mass:

sum of the atomic masses of all the atoms in a molecule or formula unit; measured in amu Example:

H 2 O

2



1 .

00794

amu



15 .

9994

amu

18 .

0153

amu

For water this can also be called the Molecular Mass

Definitions

Molar Mass –

the mass of one mole of a substance; numerically equal to the formula mass; measure in g/mol

• The formula mass of H 2 O is 18.0153 amu • The molar mass of H 2 O is 18.0153 g/mol

Sample Problems

1. What is the mass in grams of 3.04 moles of ammonia vapor, NH 3 ?

2. Calculate the mass of 0.257 moles of calcium nitrate.

3. The odor of bananas is due to the compound isopentyl acetate, C 7 H 14 O 2 . 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 grams?

Sample Problems

5. Dioxin, C 12 H 4 Cl 4 O 2 , 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 C 14 H 20 N 2 SO 4 . 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 C 3 H 4 . 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 cm 3 /4 cm 3 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 molar in mass

1

of mol of compound

 100 %

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, CuSO 4 .

5H 2 O?

3. Zinc chloride, ZnCl 2 is 52.02 % chlorine a. What mass of chlorine is contained in 80.3 g of ZnCl 2 ?

b. How many moles of Cl is this?