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Calculate percentage by mass in compounds

Quantitative chemistry • Measurements and conservation of mass

Key concepts

What you'll likely be quizzed about

  • Percentage by mass expresses the mass of an element as a percentage of the total mass of the compound.
  • Each compound contains distinct numbers of atoms of each element.
  • The proportion of an element equates to its atomic mass contribution divided by the compound's total mass, converted to a percentage.
  • Impurities and rounding of atomic masses can limit accuracy, causing the calculated percentages to deviate from theoretical values.

Flashcards

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How is percentage by mass used to check empirical formulas?

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Percentage by mass converts to mass ratios that indicate relative atom counts, which helps reveal empirical formulas.

Key notes

Important points to keep in mind

Multiply Ar by atom count to find each element's mass contribution.

Sum all element contributions to obtain the relative formula mass (RFM).

Divide element mass by RFM and multiply by 100 to get percentage by mass.

Use periodic table Ar values and specify the Ar values used when necessary.

Maintain consistent significant figures and state applied rounding rules.

Impure samples and incomplete reactions cause experimental percentages to vary from theoretical values.

Convert percentages to masses by multiplying the sample mass by the percentage (as a decimal).

Verify atom counts in the formula to avoid systematic calculation errors.

Report percentages that sum to approximately 100%; small deviations indicate rounding.

State assumptions (pure substance, accurate Ar values) when comparing theoretical calculations to experimental results.

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