Chapter 3.2: Determining the Formula of an Unknown Compound

Flashcards from Chapter 3.2 of Chemistry: The Molecular Nature of Matter and Change.

Empirical formula

Chemical formula that shows the lowest relative number of atoms

• Several different compounds can have the same one of this

Molecular formula

Chemical formula that shows the actual numbers of each element

Structural formula

Chemical formula that shows the relative placement of atoms

Isomers

Compounds with the same molecular formula but different atomic arrangements and properties

• Determine the mass of each element

• Convert each mass to amounts, writing a preliminary formula

• Convert the amounts to whole-number subscripts, by:

  • Dividing each subscript by the smallest subscript

  • Multiplying by the smallest integer that turns all subscripts into integers

How would I find the empirical formula of an unknown compound, given the masses of each element?

Divide the molar mass by the current mass of the empirical formula, then multiply by that whole-number multiple.

How would I find the molecular formula of an unknown compound, given the molar mass and empirical formula?

Assume 100.0 grams of compound, then proceed to find the empirical and molecular formula.

How would I find formulae when only given mass percents and molecular mass?

Combustion analysis

Determining a chemical formula from combustion product measurements, typically of organic compounds

• Burn a substance in excess O₂, producing CO₂ and H₂O from the carbon and hydrogen in the substance.

• Measure the amounts of CO₂ and H₂O collected.

• From the amounts of CO₂ and H₂O, calculate the masses of C and H.

  • If including a third element, subtract the masses of C and H to find the third element’s mass.

• Calculate the empirical, then molecular, formula.

How would I determine the formulae of a substance through combustion analysis?