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23 February, 08:17

terephthalic acid used in the production of polyester fibers and films is composed of carbon, hydrogen, and oxygen. When 0.6943 h og acid is subjected to combustion analysis it produced 1.471 g CO2 and 0.391 g h20. What is its empirical and molecular formula if its molar mass is 172 g/mol

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  1. 23 February, 08:46
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    Empirical formula: C₂H₃O

    Molecular formula: C₈H₁₂O₄

    Explanation:

    After the reaction, all the carbon will form CO₂ and all the hydrogen will form H₂O, so the number of moles of each element will be the same as in the terephthalic acid. Knowing the molar masses: C = 12 g/mol, H = 1 g/mol, and O = 16 g/mol. CO₂ = 12 + 2x16 = 44 g/mol, H₂O = 2x1 + 16 = 18 g/mol.

    The number of the moles of the compounds are the mass divided by the molar mass so:

    CO₂ = 1.471/44 = 0.0334 mol

    H₂O = 0.391/18 = 0.0217 mol

    There is 1 mol of C in each mol of CO₂, so there is 0.0334 mol of C.

    There are 2 mol of H in each mol of H₂O, so there is 0.0434 mol of H.

    Which are the same of in the acid. The masses of the compounds are:

    mC = 0.0334x12 = 0.400 g

    mH = 0.0434x1 = 0.0434 g

    mO = 0.6943 - (0.4 + 0.0434) = 0.2509 g

    The number of moles of O is: 0.2509/16 = 0.0157 mol

    To know the empirical formula, let's divide the number of moles of the elements for the samllest: 0.0157 mol

    C = 0.0334/0.0157 = 2

    H = 0.0434/0.0157 = 3

    O = 0.0157/0.0157 = 1

    The empirical formula is C₂H₃O

    The molecular formula will be the empirical multiplied by a factor (n):

    n (C₂H₃O), so the molar mass will be: nx (2x12 + 3x1 + 16) = 172

    43n = 172

    n = 4

    So the molecular formula is 4x (C₂H₃O) = C₈H₁₂O₄.
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