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Anonymous
Anonymous asked in Science & MathematicsChemistry · 1 decade ago

What is the molecular formula of the compound?

A gaseous hydrogen and carbon containing compound is decomposed and found to contain 85.63% C and 14.37% H by mass. The mass of 258mL of the gas, measured at STP, was found to be 0.646g. What is the molecular formula of the compound?

Update:

Thank You!!!

2 Answers

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  • Lexi R
    Lv 7
    1 decade ago
    Favorite Answer

    first work out the empirical formula by determining how many moles of each atom are in a sample of the compound.

    Then use the volume at STP to work out moles of gas, and thus molar mass of the compound.

    Then you can find the molecular formula from the molar mass and empirical formula

    Start with 100 g, work out moles of each atom in the 100 g

    mass H = 14.37 g

    moles H = 14.37 g / 1.008 g/mol = 14.26 mol

    mass C = 85.63 g

    moles C = 85.63 g / 12.01 g/mol = 7.13 moles

    Ratio of C : H

    7.13 mol : 14.26 mol

    Divide both numbers in the ratio by the smallest number

    C : H

    7.13 / 7.13 : 14.26 / 7.13

    = 1 : 2

    So empirical formula is

    CH2

    work out the moles of the gas form the volume at STP

    STP is standard Temp and pressure, P = 1 atm and T = 0 deg C

    At STP 1 mole of any ideal gas occupies a volume of 22.4 L

    So if you know the volume at STP

    moles = volume (L) / 22.4 L/mol

    moles Compound = 0.258 L / 22.4 L/mol

    = 0.01152 moles of compound

    You now know that

    moles = 0.01152 mol

    and mass = 0.646 g

    So you can find molar mass

    moles = mass / molar mass

    therefore molar mass = mass / moles

    = 0.646 g / 0.01152 mol

    = 56.1 g/mol

    To work out the molecular formula divide the molar mass by the mass of the empirical formula

    This tells you how many times to empirical formula fits into the molecular formula

    empirical mass = 12.01 + (2 x 1.008) = 14.026 g/mol

    molar mass / empirical mass

    = 56.1 g/mol / 14.026 g/mol

    = 4

    So molecular mass = empirical mass x 4

    4 x (CH2)

    C4H8

  • Bobby
    Lv 7
    1 decade ago

    It best to work out the molecular weight then the molecular formula first

    1 mole occupies 22.4 liters ; in .285 liters we have .258 / 22.4 = 0.0115 moles

    these weigh .646 g so 1 mole weighs .646 / .0115 = 56.08 g/mole

    to calculate the composition of the compound we have

    85.63% C or 86.63 % of 56.08 = 48g or 4 moles of carbon

    14.37 % H or = 14.34 % of 56.08 = 8.04 g or moles of H

    molecular formula is C4H8 ... simple ..

    N B :you don't need to calculate the empirical formula first.

    also the empirical mass is not a real chemical entity ??

    in my solution we are only working with real chemical experimental data .. good luck ,,

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