The empirical formula is #"C"_3"H"_8"#.
Since the percentages add up to 100, we can assume that we have a 100.0 g sample of the compound, and the percentages become grams.
Determine the Moles of Each Element
Divide the mass of each element by its molar mass. The molar mass of an element is its atomic weight on
the periodic table in grams/ mole (g/mol).
#81.82cancel"g C"xx(1"mol C")/(12.011cancel"g C")="6.812 mol C"#
#18.18cancel"g H"xx(1"mol H")/(1.00794cancel"g H")="18.04 mol H"#
Determine the Mole Ratios
Divide the number of moles of each
element by the least number of moles.
#"C":##(6.812)/(6.812)="1.000"#
#"H":##(18.04)/(6.812)="2.648"#
Since the mole ratio for H cannot be rounded to a whole number, we must multiply it times a factor that will result in a whole number.
#"H":##2.648xx3=7.944~~8#
We must multiply both mole ratios times #3#.
#"C":##1.000xx3=3.000"#
The empirical formula is #"C"_3"H"_8"#.
Given the percentage composition of HC as #C->81.82% and H->18.18%#
So the ratio of number if atoms of C and H in its molecule can will be
#C:H=81.82/12:18.18/1#
#C:H=6.82:18.18#
#=6.82/6.82:18.18/6.82=1:2.66~~3:8#
So the Empirical Formula of hydrocarbon is #C_3H_8#
As the mass of one litre of hydrocarbon is same as that of #CO_2# The molar mass of the HC will be same as that of #CO_2# i.e #44g/"mol"#
Now let Molecular firmula of the HC be #(C_3H_8)_n#
Using molar mass of C and H the molar mass of the HC from its molecular formula is
#(3xx12+8xx1)n=44n#
So
#44n=44=>n=1#
Hence the molecular formula of HC is #C_3H_8#
Home > Community > A compound contains 81.8% C and 18.2% H by mass. What is the empirical formula?
C 81.8/12= 7
H 18.2/1=18.2
Take the smallest ratio and divide all through
C 7/7=1
H 18.2/7=3
1:3
Empirical formular CH3 which is Methyl
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C 81.8/12= 7
H 18.2/1=18.2
Take the smallest ratio and divide all through
C 7/7=1
H 18.2/7=3
1:3
Empirical formular CH3 which is Methyl
Number of carbon atoms : Number of hydrogen atoms =
(mass percent of carbon/Atomic mass of carbon) : Mass percent of hydrogen/Atomic mass of hydrogen)
= 81.8/12 : 18.2/1.008 = 6.8167 : 18.0556 = 1 : 2.6487 = 3 : 7.95 = 3 : 8 (nearest positive integer)
So, the empirical formula of the compound is C3H8.
Hope, this helps.
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Number of carbon atoms : Number of hydrogen atoms =
(mass percent of carbon/Atomic mass of carbon) : Mass percent of hydrogen/Atomic mass of hydrogen)
= 81.8/12 : 18.2/1.008 = 6.8167 : 18.0556 = 1 : 2.6487 = 3 : 7.95 = 3 : 8 (nearest positive integer)
So, the empirical formula of the compound is C3H8.
Hope, this helps.
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