General chemistryLesson 2 of 2 in the course Composition of Substances and Solutions

Percent Composition and Empirical-to-Molecular Formula Calculations

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

A compound’s percent composition describes how much of its total mass comes from each element. It is useful for identifying substances, comparing compounds, and beginning the process of determining an unknown chemical formula.

%X=mass of Xmass of compound×100%\% X = \frac{\text{mass of } X}{\text{mass of compound}} \times 100\%

The numerator is the measured mass of the particular element, and the denominator is the mass of the entire sample. The same calculation is done separately for every element in the compound.

All flashcards

Empirical formula
A formula showing the smallest whole-number ratio of atoms in a compound.
Molecular formula
A formula showing the actual number of each type of atom in a molecule.
Formula mass
The sum of the atomic masses represented by a chemical formula.
Percent composition by mass
The percentage of a compound’s total mass contributed by each element.
Assuming a 100-g sample
A method that turns each mass percentage into the same numerical mass in grams.

Quiz questions

  1. For ammonia, NH3NH_3, what is the approximate mass percentage of nitrogen? Use atomic masses N = 14.01 and H = 1.008.Answer: 82.3%
  2. When deriving an empirical formula from measured masses of elements, what should be done first?Answer: Convert each element’s mass to moles using its molar mass.
  3. A mole ratio is 1:1.51:1.5. What whole-number ratio results from multiplying every value by the same integer?Answer: 2:32:3
  4. A compound has an empirical formula mass of 30 g/mol and a molar mass of 180 g/mol. What is the multiplier nn?Answer: 66
  5. What is the approximate formula mass of NH3NH_3? Use atomic masses N = 14.01 and H = 1.008.Answer: 17.03 amu

Based on Chemistry 2e by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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