Question

A 40.4 g sample of a protein contains 17.16 g of carbon, 3.17 g of hydrogen, 13.71 g of oxygen, and the rest being nitrogen. This 40.4-gram sample is known to be 0.07141 moles. Determine the molecular formula of this protein.

Answer 1

The molecular formula of this protein : C₁₈H₄₂O₁₂N₆

Further explanation

The empirical formula is the smallest comparison of atoms of compound forming elements.

A molecular formula is a formula that shows the number of atomic elements that make up a compound.

(empirical formula) n = molecular formula

The principle of determining empirical formula and molecular formula

• Determine the mass ratio of the constituent elements of the compound.
• Determine the mole ratio by dividing the elemental mass with the relative atomic mass obtained by the empirical formula
• Determine molecular formulas by looking for values of n

Find mol ratio for every component :

• C (Ar=12 g/mol):

`\tt (17.16)/(12)=1.43`

• H(Ar=1 g/mol) :

`\tt (3.17)/(1)=3.17`

• O(Ar=16 g/mol) :

`\tt (13.71)/(16)=0.857`

N (r=14 g/mol) :

Mass of Nitrogen :

40.4-(17.16+3.17+13.71)=6.36 g

• N :

`\tt (6.36)/(14)=0.454`

C : H : O : N = 1.43 : 3.17 : 0.857 : 0.454 = 3.15 : 7 : 1.89 : 1=3:7:2:1

Empirical formula : C₃H₇O₂N

Molecular mass of protein :

`\tt (40.4)/(0.07141)=565.75`

(C₃H₇O₂N)n=565.75

(12.3+1.7+2.16+14)n=565.75

(89)n=565.75

n=6.4≈6

so the molecular formula : C₁₈H₄₂O₁₂N₆