IB Chemistry-Empirical Formula

Introduction to Mole Calculations

• When tasked with determining moles from mass, we use the given mass divided by molar mass.

• This process differs from traditional ways since the mass won't sum to a hundred.

Identifying the Compound

• Lead is represented by the symbol Pb (not "PV").

• Molar masses are acquired from the periodic table.

Calculating Moles

• After acquiring molar mass, divide the sample mass by the molar mass to find the number of moles for each element.

• Identify which element has the smallest number of moles based on calculations.

• Perform the "divide by small" step: divide all mole amounts by the smallest number of moles found.

Finding Ratios

• The resulting ratio helps determine the composition of the compound.

• The smallest number will always be one when dividing itself (e.g., if the smallest number is 3, then the ratio might yield results like 1 : 3).

• Ratios may not always yield whole numbers, which brings us to the next step.

Multiply to Whole Numbers

• In cases where ratios result in fractional values:

  • If it ends in .5, multiply the entire ratio by 2.

  • If it ends in .333 (or .666), multiply by 3 for conversion into whole numbers.

• Example operation: a ratio of 1 : 3 : 1.333 would lead us to multiply by 3 for conversion to whole numbers, achieving a ratio of 3 : 9 : 4.

Resulting Molecular Formula

• This ratio reflects the molecular formula of the compound:

  • C3H9O4.

• Calculate the molar mass of this compound:

  • C: 3 x 12g/mol = 36g/mol

  • H: 9 x 1g/mol = 9g/mol

  • O: 4 x 16g/mol = 64g/mol

  • Total molar mass = 36 + 9 + 64 = 109 g/mol.

Conclusion

• Complete understanding of mole calculations is vital for identifying the molecular structure based on mass and elemental ratios.