2.2 c: Percentage Composition and Empirical Formula

Percentage Composition and Empirical Formula

Year 11

​What we will cover

• Definitions

• ​Percentage composition calculations

• Empirical formula calculations

• Molecular formula calculations

• Mass calculations from chemical equations ​

​Revision of Module 2 Basics

• Law of conservation of mass

• ​Molar mass

• Converting between moles, particles, and mass ​

Multiple Choice

2H₂  +   O₂  →  2H₂O
How many moles of oxygen are consumed if 8 moles H₂ are used?

• 2
• 4
• 6
• 8

Multiple Choice

The mass, in atomic mass units, of O₂ is?

• 16 amu
• 16 g
• 32 g
• 32 amu

Multiple Choice

There are ______ number of molecules of NaCl in one mole of NaCl?

• 6.02 x 10²³
• 1 mole
• 2
• Mr. Miller

Multiple Choice

What is the mass of 1.2 x 10²⁴ atoms of C?

• 2.0 grams

• 24 grams

• 0.17 grams

• 1.4 x 10²⁵ grams

​Percentage Composition

Fe₂O_3​

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How much iron do we have?

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How much oxygen do we have? ​

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​Percentage Composition Calculation

​Calculate the percentage of iron in iron oxide

Open Ended

Calculate the percentage of nitrogen in the fertilizer ammonium sulfate (NH₄)₂SO₄

​Empirical Formula

1. ​Determine masses

2. Determine moles ​

3. Divide through by smallest number

4. Round off to get closest whole number ​

• MF is always a whole number multiple of the EF

• EF gives you a simplified/reduced ratio of the atoms in a compound, the MF gives you the actual amount of atoms in the compound

​Calculating empirical formula

​4.97g of lithium metal reacted with oxygen, and formed 10.89g of lithium oxide. Calculate the formula of the compound.

Open Ended

0.84g of zinc was reacted with chlorine gas. The mass of zinc chloride formed was 1.75g. Calculate the formula of the compound formed.

​Calculating molecular formula

​Using empirical formula and molecular mass:

Naphthalene is a compound containing carbon and hydrogen that is often used in mothballs. Its empirical formula is C5H4 and its molar mass is 128.16 g/mol. What is its molecular formula?

​Mass Calculations from Chemical Equations

How can we determine how much of a particular reactant we need?

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Or how much product that can be formed given the mass of a reactant? ​

Calculate the mass of hydrochloric acid needed to dissolve 5.0g of magnesium carbonate. ​

Open Ended

​Next up:

• Limiting reagent/reactant calculations

• 2.3 ​Concentration calculations