Elemental Composition of Pure Substances

AP Chemistry· difficulty 2/5

A 0.250 g sample of a hydrocarbon is burned completely in excess O2. The combustion products are passed through a dehydrating tube (gains 0.306 g) followed by a CO2 absorber (gains 0.747 g). The hydrocarbon contains only C and H.

furnace H2O abs +0.306 CO2 abs +0.747 sample 0.250 g hydrocarbon

What is the empirical formula?

  • A

    CH3

  • B

    CH

  • C

    C2H3

  • D

    CH2

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Explanation

mol C = 0.747/44.01 = 0.01698; mass C = 0.2039 g. mol H = (0.306/18.02)*2 = 0.03396; mass H = 0.0342 g. Sum = 0.238 g (consistent with 0.250 within error). Ratio C:H = 0.01698:0.03396 = 1:2 -> CH2.

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