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7 November, 10:12

5.50g of a certain Compound X, known to be made of carbon, hydrogen and perhaps oxygen, and to have a molecular molar mass of 136./gmol, is burned completely in excess oxygen, and the mass of the products carefully measured: product mass carbon dioxide 14.24g water 2.91g Use this information to find the molecular formula of X.

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  1. 7 November, 13:44
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    The molecular formula is C8H8O2

    Explanation:

    Step 1: Data given

    Mass of compound X = 5.50 grams

    Mass of CO2 = 14.24 grams

    Molar mass of CO2 = 44.01 g/mol

    Mass of H2O = 2.91 grams

    Molar mass H2O = 18.02 g/mol

    Molar mass C = 12.01 g/mol

    Molar mass O = 16.0 g/mol

    Molar mass H = 1.01 g/mol

    Molar mass of the compound = 136 g/mol

    Step 2: Calculate moles CO2

    Moles CO2 = 14.24 grams / 44.01 g/mol

    Moles CO2 = 0.324 moles

    Step 3: Calculate moles C

    For 1 mol cO2 we have 1 mol C

    For 0.324 moles CO2 we have 0.324 moles C

    Step 4: Calculate mass C

    Mass C = 0.324 moles * 12.01 g/mol

    Mass C = 3.89 grams

    Step 5: Calculate moles H2O

    Moles H2O = 2.91 grams / 18.02 g/mol

    Moles H2O = 0.161 moles

    Step 6: Calculate moles H

    For 1 mol H2O we have 2 moles H

    For 0.161 moles H2O we have 2*0.161 = 0.322 moles H

    Step 7: Calculate mass H

    Mass H = 0.322 moles * 1.01 g/mol

    Mass H = 0.325 grams

    Step 8: Calculate mass O

    Mass O = 5.50 grams - 3.89 grams - 0.325 grams

    MAss O = 1.285 grams

    Step 9: Calculate moles O

    Moles O = 1.285 grams / 16.0 g/mol

    Moles O = 0.0803 moles

    Step 10: Calculate mol ratio

    We divide by the smallest amount of moles

    C: 0.324 moles / 0.0803 moles = 4

    H: 0.322 moles / 0.0803 moles = 4

    O: 0.0803 moles / 0.0803 moles = 1

    The empirical formula is C4H4O

    The molar mass of this empirical formula is 68 g/mol

    Step 11: Calculate the molecular formula

    We have to multiply the empirical formula by n

    n = 136 g/mol / 68 g/mol

    n = 2

    We have to multiply the empirical formula by 2

    Molecular formula = 2 * (C4H4O) = C8H8O2

    The molecular formula is C8H8O2
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