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A 9.86-g sample of a gaseous compound containing only carbon and hydrogen contains 8.44 g of carbon. At STP, a 278 mL sample of the gas has a mass of 3.48×10-1 g. What is the molecular formula of the compound?

I'm not sure what formula to use or how to get started.....

Please help!

Thank you so much

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  • 8 years ago
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    First figure out the empirical formula using the information from the first sentence

    Subtract sample in grams from amount in g of carbon to find amount in g of hydrogen

    9.86 g sample – 8.44 g C = 1.42 g H

    Convert grams of hydrogen and carbon to moles

    H = 1.42 g H x (1 mole / 1.00794 g H) = 1.4088 mole H

    C = 8.44 g C x (1 mole / 12.0107 g C) = 0.7027 mole C

    Divide by smallest amount of moles (0.7027)

    C = 0.7027 / 0.7027 = 1

    H = 1.4088 / 0.7027 = 2

    EMPIRICAL FORMULA = CH2

    Now find molecular formula using the ideal gas law with information in the second part of the sentence. (STP means standard temp of 273 K and pressure = 1 atm)

    PV = nRT

    P = 1 atm

    V = 0.278 L

    n = PV / RT = 0.01241 mol

    R = 0.08206 L atm/mol K

    T = 273 K

    Your sample of gas has a mass of 3.48 x 10-1 g

    Molar mass (g/mol) = 3.48 x 10-1 g / 0.01241 mol = 28.04 g/mol

    Divide this by the molar mass of CH2

    28.04 / 14.03 = 2

    Multiply this by the empirical formula to get the molecular formula of C2H4 or ethylene

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