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Worksheet 1B: Formulas and Masses

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    1. Fill in the chart below, using a periodic table as the only source of additional information.

    Compound (Molecular Formula) Empirical Formula Molar Mass Moles in 100.0 g

    2. Calculate the percentage composition by mass of each of the compounds which are the same as in Question #1. The first compound has been completed as an example.

    Compound Percentage Composition by Mass
    \(H_2O\) 88.8% O, 11.2% H

    3. Chemical analysis commonly produce weight percent elemental data of the elements present in a pure sample of a compound. An analysis of pyrophosphoric acid produces the following data: 2.27% hydrogen, 34.80% phosphorus, and the remainder is oxygen.

    a) What is the mass of each element present in a 100.0 g sample? (You may wish to divide your group into pairs and assign one element to each pair.)

    H _____ P _____ O _____

    b) Determine the number of moles of each element in the 100.0 g sample.

    H _____ P _____ O _____

    c) Find the simplest whole-number ratio of the number of moles of each element. This can be found by dividing the number of moles of each element by the number of moles of the element with the smallest number of moles. Your result gives the empirical formula of pyrophosphoric acid.

    H _____ P _____ O _____

    d) The molar mass of pyrophosphoric acid is 177.97 g/mol. What is its molecular formula?

    4. Vitamin C is an antioxidant. This class of compounds is important in biochemistry partly because antioxidants negate the effect of potentially harmful oxidizing substances in the body such as singlet oxygen and free radicals

    Actual elemental composition data is developed from combustion of a pure sample. A 35.5 mg sample of vitamin C was placed in a combustion apparatus, and 53.3 mg \(CO_2\) and 14.4 mg \(H_2O\) were recovered. Try using a group brainstorming approach to solve parts (a) through (d).

    a) Write an unbalanced chemical equation, identifying the reactants and products. The equation cannot yet be balanced because vitamin C can only be represented as \(C_xH_yO_z\), where \(x\), \(y\), and \(z\) are integers to be determined in a later step in the problem solution.

    b) What is the reactant source of the carbon in the carbon dioxide product?


    c) What is the reactant source of the hydrogen in the water product?


    d) What is the reactant source of the oxygen in the carbon dioxide and water products?


    Try using a group round robin approach to solve the remaining parts of the question.

    e) How many moles of carbon were in the original vitamin C sample?

    f) How many moles of hydrogen were in the sample?

    g) How many moles of oxygen were in the sample?

    h) What is the mole ratio of carbon to hydrogen to oxygen in vitamin C?

    i) What is the empirical formula of vitamin C?

    j) The molar mass of vitamin C is 176 g/mol. What is its molecular formula? In other words, what are the values of \(x\), \(y\), and \(z\) in \(C_xH_yO_z\)?

    Worksheet 1B: Formulas and Masses is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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