Practice Problems: Chemical Formulas (Answer Key)


  1. H3PO4, Phosphoric acid, is used in detergents, fertilizers, toothpastes and flavoring in carbonated beverages. Calculate the percent composition by mass to two decimal places of H, P and O in this compound.

    H: 3.09%, P: 31.61%, O: 65.31%

  2. What is the percent composition by mass of aspartame (C14H18N2O5), the artificial sweetener NutraSweet?

    C: 57.14%, H: 6.12%, N: 9.52%, O: 27.21%

  3. Ascorbic acid (vitamin C) is 40.92% C, 4.58% H and 54.50% O by mass. What is the empirical formula of ascorbic acid?

    C3H4O3

  4. What is the empirical formula of each of the following compounds?

    a. Talc by mass composition contains 19.2% Mg, 29.6% Si, 42.2% O and 9.0% H.  Mg3Si4O10H34

    b. Saccharin has by mass composition 45.89% C, 2.75% H, 7.65% N, 26.20% O and 17.50% S.   C7H5NO3S

    c. Salicylic Acid, used in aspirin, contains 60.87% C, 4.38% H, and 34.75% O by mass composition.  C7H6O3

    d. L-Dopa, a drug used for the treatment of Parkinson's disease, is 54.82% C, 5.62% H, 7.10% N, and 32.46% O by mass composition.  C9H11NO4


  5. Determine the empirical formula of the following compounds that underwent combustion analysis.

    a. Toluene is composed of C and H and yields 5.86 mg of CO2 and 1.37 mg of H2O after combustion.  C7H8

    b. 0.1005 g of menthol, which is composed of C, H, and O, yields 0.2829 g CO2 and 0.1159 g H2O after combustion.  C10H20O


  6. What is the molecular formula of benzoyl peroxide (the empirical formula is C7H5O2) if the molecular mass is 242 g/mol?  C14H10O4


  7. What are the empirical and molecular formulas of the following compounds?

    a. Ibuprofen by mass composition is 75.69% C, 8.80% H and 15.51% O and the molecular mass is approximately 206 g/mol.  C13H18O2

    b. Caffeine contains by mass composition 49.5% C, 5.15% H, 28.9% N and 16.5 % O and the molecular mass is about 195 g/mol.  C4H5N2O, C8H10N4O2