ASP - Percent Composition and Empirical & Molecular Formulas (Lesson)

Quantitative Consideration of Mixtures

Quantitatively, the composition of a mixture can be used to determine the relative amounts of the pure substances that compose it. Elemental analysis involves finding the relative mass of one or more elements in the mixture and is often used by chemists to determine how pure a substance is.

View the following video to learn how to calculate the percentages in a mixture. 

How to Calculate Percent Composition

CALCULATING PERCENT COMPOSITION

Calculating Empirical Formula From Mass Data

From experimental data, the mass of each element in a sample can be determined and shown as a percentage of the total. These percentages can be used to establish the empirical formula for the compound in four calculation steps.

  1. Use percent as a mass to convert to moles.
  2. Divide all mole amounts by the lowest number of moles.
  3. Multiply each of the divided mole values by a whole number so that each are whole numbers (if necessary).
  4. Use these whole numbers as subscripts to write the empirical formula.

Please watch the video below to learn more about calculating empirical formulas.

Molecular Formula

Empirical formulas of substances can be useful; however, they only give information regarding the ratio of each element in a compound. They do not tell us the precise number of each element. The proportions by mass are the same, but the number of atoms making up the molecule differs. For example, consider nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4). The chemical properties of these two compounds are different, but the lowest whole number of atoms of each element in a compound is the same. This is known as the empirical formula for the compound, in this case, NO2. Whereas a molecular formula only gives a ratio of each element, a molecular formula gives the exact number of each element present in a compound.

Another example is formaldehyde and glucose. Both share the empirical formula CH2O. The empirical formula is the same as the molecular formula for formaldehyde, CH2O, but for glucose, there are 6 carbons, 12 hydrogens, and 6 oxygens in each molecule resulting in a molecular formula of C6H12O6. The molecular mass of glucose would be 180 grams/mole and for formaldehyde molecular mass is 30. The ratio of glucose molecular mass to formaldehyde would be 180/30 or 6:1. Multiplying the empirical formula for glucose by six gives the molecular formula.

CALCULATING MOLECULAR FORMULA

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