Elemental analysis of a compound showed that it consisted of 81.82% carbon and 18.18% hydrogen by mass, how many hydrogen atoms appear in the empirical formula of the compound?

1 Answer
Nikka C.
Nov 12, 2015

8 #H# atoms

Explanation:

For this kind of problem, we have to assume that you have 100g unknown sample (since the percentages add up to 100%).

Thus, masses are C = 81.82 g; H = 18.18 g

Since chemical formulas deal with number of moles more than their respective weights, we need to multiply the masses with their respective atomic weights to get the number of moles.

#C# = #81.82 cancel g# x #"1 mol"/(12.01 cancel g)# = 6.81 mol #C#

#H# = #18.18 cancel g# x #"1 mol"/(1.01 cancel g)# = 18 mol #H#

Next, to get the ratio of atoms to each other, we would need to divide the number of moles by the smallest number of mole.

#C# = #(6.81 cancel "mol")/(6.81 cancel "mol")# = 1

#H# = #(18.18 cancel "mol")/(6.81 cancel "mol")# = #color (red) (2.64)#

Now, since the number of #H# atoms is too far to round off, we need to find a factor that we can multiply to BOTH atoms to get the accurate ratio. In this case, the factor is 3.

#C# = 1 x 3 = #3#

#H# = 2.64 x 3 = #7.92~~ 8#

Therefore, the empirical formula is #C_3H_8#.

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