Chapter 7 Section 4.1

Chapter 7 Section 4: Determining Chemical Formulas

Multiple Choice

Review Question: Which of the following compounds does not contain a polyatomic ion?

• Sodium Carbonate

• Sodium Sulfate

• Sodium Sulfite

• Sodium Sulfide

​Introduction

• ​When a new substance is discovered, we analyze it to reveal the percentage composition

• ​Using this information we can determine an elements empirical formula

  • ​Empirical formula consists of the symbols for the elements combined in a compound, with the subscripts showing the smallest whole number ratio of the different atoms in the compound

    • ​the simplest ratio of atoms

    • ​For an ionic compound this is the chemical formula

    • ​for molecular compound it may not always be the molecular formula

      • ​Diborane gase

        • ​Molecular formula: B₂H_6. Empirical formula: BH₃

Multiple Choice

The molecular formula for acetylene is C₂H₂. The molecular formula for benzene is C₆H₆. Which of the following is the empirical formula for both

• CH

• C₂H₂

• C₆H₆

• (CH)₂

​Calculation of Empirical Formulas From Percentage composition

​Calculation of Empirical Formulas From Percentage composition

​Practice problem L page 246

• ​Quantitative analysis shows that a compound contains 32.38% sodium, 22.65% sulfur, and 44.99% Oxygen. Find the empirical formula

  • ​First step: go from percentage composition to mass composition

    • ​32.38% Na = 32.38 g Na

    • ​22.65% S = 22.65 g S

    • ​44.99% O = 44.99 g O

  • ​Second step: convert mass to moles

​​Practice problem L page 246