The compound adrenaline contains 56.79% carbon, 6.56% hydrogen, 28.3% oxygen, and 8.28% nitrogen by mass, what is its empirical formula?
Moles = mass/molar mass
First, as each percentage is over 100, multiply the values of carbon, hydrogen, oxygen and nitrogen by 100. You will obtain:
56.79 grams of carbon
6.56 grams of hydrogen
28.3 grams of oxygen
8.28 grams of nitrogen
Moles of carbon: 56.79/12.0107 = 4.728 moles
Moles of hydrogen: 6.56/1.00794 = 6.51 moles
Moles of oxygen: 28.3/15.9994 = 1.77 moles
Moles of nitrogen: 8.28/14.0067 = 0.59 moles
Divide all moles by the smallest value. In this case, it's 0.59 moles.
4.728/0.59 = 8
6.51/0.59 = 11
1.77/0.59 = 3
Thus, the emperical formula is C8H11NO3
With this kind of problem, it is always safe to assume that you have 100g sample. It makes the mathematical equation easier.
Assume 100g sample. Therefore,
wt of C = 56.79 g
wt of H = 6.56 g
wt of O = 28.30 g
wt of N = 8.28 g
Since molecular and empirical formulas deal with number of moles rather than weight, you need to convert these number of grams into number of moles. How? By multiplying the weight with the corresponding atomic weight.
Get the element with the smallest number of moles, (in this case, N) and use it as your denominator in determining the number of atoms for the other elements.
Therefore, the empirical formula for adrenaline is