Question

A compound with the molar mass of approximately 142 g/mol has the composition 50.7% C, 9.9% H, and 39.4% N. What is its molecular formula?
Element order CxHyNz.

Answer 1

50.7% C, 9.9% H, and 39.4% N. If you take a 100 g sample you will have
50.7 g C, 9.9 g H and 39.4 g N.
mols C = 50.7/12 = about 4.2 but you can use more exact numbers here and below
mols H = 9.9/1 = 9.9
mols N = 39.4/14 = 2.8
To find the ratio divide by the smallest number. like this.
4.2/2.8 = 1.5 C
9.9/2.8 = 3.5 H
2.8/2.8 = 1.0 N
Since compounds are formed with WHOLE numbers and not fractions, convert them to whole numbers but keep the ratio the same. You can do that by multiplying each by 2 so you get 3 C, 7 H, and 2 N so the empirical formula is
C3H7N2 and the empirical mass will be 3*12 + 7*1 + 2*14 = 71
Then as I showed you below (C3H7N2)x = 142 which makes (71)x = 142 and x will be 2. The molecular formula then can be written as C6H14N4. I always like to check the molar mass to see that it is close to that 142.
6*12 = 72
14*1 = 14
4*14 = 56
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total = 142