In order to find the empirical formula of this compound, we need to find the number of moles of each element present in this molecule, we can do that by using the given information if the question, and the molar mass of each element:
90 grams of Carbon, molar mass = 12 grams/mol
11 grams of Hydrogen, molar mass = 1 gram/mol
35 grams of Nitrogen, molar mass = 14 grams/mol
Now let's start with Carbon:
12g = 1 mol
90g = x moles
x = 7.5 moles of Carbon in 90 grams
Hydrogen:
1g = 1 mol
11g = x moles
x = 11 moles of Hydrogen in 11 grams
Nitrogen:
14g = 1 mol
35g = x moles
x = 2.5 moles of Nitrogen in 35 grams
Now we have 7.5 moles of C, 11 moles of H and 2.5 moles of N, according to the rules on building the molecular formula of a compound, we can not use decimal numbers to build it, therefore we need to multiply all the values for 2
7.5 * 2 = 15 moles of Carbon
11 * 2 = 22 moles of Hydrogen
2.5 * 2 = 5 moles of Nitrogen
Now we have all the number in the integer form
To find the empirical formula, we need to divide all the values by the smallest value of moles:
15/5 = 3 moles
22/5 = 4.4 moles
5/5 = 1 mol
Therefore the final empirical formula is C3H4N