Stoichiometry

Question

Find the formula of the food enhancer MSG which has a composition of 35.51% C, 37.85% O, 8.29% N, 13.60% Na, and 4.77% H

Answer 1

C:   35.51 g C / (12.0107 g/mol C) = 2.957 mol C

O:   37.85 g O / (15.9994 g/mol O) = 2.366 mol O

N:   8.29 g N / (14.0067 g/mol N) = 0.5919 mol N

Na:   13.60 g Na / (22.990 g/mol Na) = 0.5916 mol Na

H:   4.77 g H / (1.008 g/mol H) = 4.732 mol H

 

Lowest number of moles: 0.5916, N

So, 

C:   2.957/0.5916 = 4.998 ≈ 5

O:   2. 366/0.5916 = 3.999 ≈ 4

N:   0.5919/0.5916 = 1.001 ≈ 1

Na:   0.5916/0.5916 = 1

H:   4.732/0.5916 = 7.999 ≈ 8

 

This gives us C₅H₈NO₄Na.

Answer 2

When given % composition, it is easiest to assume you have 100 g, and then the % of each element is in grams. Then convert to moles to get empirical formula, as follows:

35.51 g C x 1 mole/12 g = 2.96 moles C

37.85 g O x 1 mole/16 g = 2.37 moles O

8.29 g N x 1 mole/14 g = 0.592 moles N

13.60 g Na x 1 mole/23 g = 0.591 moles Na

4.77 g H x 1 mole/1 g = 4.77 moles H

Dividing through by 0.591 one obtains

C = 5

O = 4

N = 1

Na = 1

H = 8

So, empirical formula is C₅H₈NO₄Na.  This happens to be the molecular formula as well.