Molar Volume of Gases, Percent
Composition, Empirical/ Molecular
Formula
Finishing Up Ch. 10

Check it out!
• In 1811, Avogadro proposed a hypothesis:
– Volumes of gases at the same temp and pressure have the same number of particles
• https://www.youtube.com/watch?v=CKs4WKv
6foI

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• Volumes of 1 mole of different solid or liquid substances are not the same.
• Gases are a different story!
• With varying temperature, volume of gases change. But at STP (0 deg C and 1 atm), 1 mol of ANY gas, has a volume of 22.4 L.
• Molar volume of a gas = 22.4 L

Calculating the Volume of a Gas at STP
Volume of a gas = Moles of gas X 22.4 L
1 mole

Sulfur dioxide is a gas produced by burning coal.
It is an air pollutant and one of the causes of acid rain. Determine the volume, in L, of 0.60 mol sulfur dioxide gas at STP.

• What is the volume of these gases at STP?
– 3.20 x 10 -3 mol CO
2
– 3.7 mol N2

When a 13.60 g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained.
What is the percent composition of this compound?

Calculate the Percent Composition by
Mass of Sulfur in Calcium Sulfite

• The empirical formula shows the lowest whole number ratio of atoms in a compound.
• Ex: Hydrogen peroxide

- Find moles of each component by using each one’s mass
- Divide each by smallest number of moles
- Multiply each if necessary to obtain whole numbers

• A compound is analyzed and found to contain
25.9% nitrogen and 74.1% oxygen. What is the empirical formula for the compound?

1,6 diaminohexane is used to make nylon. What is the empirical formula of this compound if it is 62.1% C, 13.8% H, and 24.1% N?

• The molecular formula of a compound is either the same as its empirical formula, or it is a simple whole-number multiple of its empirical formula.

To find Molecular formula:
- Find molar mass of the empirical formula
- Divide the given molar mass by the empirical molar mass to get a whole number
- Finally, multiply this whole number by each subscript in the formula

• Calculate the molecular formula of a compound whose molar mass is 60.0 g/mol and empirical formula is CH
4
N.

• Find the molecular formula of ethylene glycol
(antifreeze). The molar mass is 62 g/mol and the empirical formula is CH
3
O.

HW- practice 32-37