AP Chemistry

A 2.5 gram sample of a hydrocarbon containing C, H, N, O is combusted in excess oxygen. The nitrogen compounds that are formed are treated to convert all of N to N2. The gases are passed through a CaCl2 drying tube which weighs 1.00 gram initially and 3.16 grams after the gases are passed through the tube. The remaining gas stream is bubbled into water where the CO2 is converted to H2CO3. Titration to equivalence requires 513mL of 0.4M NaOH. The O2 is removed by reacting with hot copper metal. The remaining N2 is collected in a 250mL container where it has a pressure of 1273 Hg at 25C. In a separate experiment, 36.5 grams of the compound is dissolved in 500 grams of CCl4 (Kf=29.8C/m) and the freezing point of the CCl4 is lowered 14.9%.

a- Determine the masses of C, H, N, O
b- Determine the empirical formula
c- Determine the molecular formula

I do not know where to start. Anything would be helpful. Thanks

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  1. P.S. don't forget the Related Questions below.


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  2. CaCl2 absorbs H2O (H2O comes from the H); the difference in mass is grams H2O. Convert that to grams hydrogen.

    Carbon is converted to CO2 then to H2CO3 and titrated. From the titration data yu obtain grams C.

    Use PV = nRT to solve for n for nitrogen and convert to grams.

    Oxygen is obtained from 2.5 g minus the sum of the masses of C, H, N.

    That gives you part a.
    Part b.
    Convert grams of each to moles (by dividing bv the atomic mass of the element), then find the ratio (in small whole numbers) of the elements to each other. The easy way to do that is to divide the smallest number by itself (making it 1.00) and dividing all of the other numbers by the same small number. That give you part b.

    Part c.
    Use delta T = Kf*m and solve for molality.

    m = moles/kg solvent.
    Solve for moles.

    moles = grams/molar mass
    solve for molar mass.

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  3. Thanks so much! Very thorough

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