Question

9) A unknown sample is run thru a Mass Spec and comes out with a 40.1% sulfur and 59.9% oxygen content. Determine the empirical formula of the unknown.


10) The substance in milk that gives sour milk its taste is lactic acid. it consists of 40.0% carbon, 6.71% hydrogen and the rest oxygen. Determine its empirical formula.


11) The empirical formula for a compound is SCl, with a molar mass of 135g/mol. Determine the Molecular formula of the compound.

Answer 1

10)

90g/mol is 100%

for C --40% is 90*40/100=36 (36/12 is 3 C3)

for H = 6

(6/1=6 H6)

for O =48 (48/16= 3 O3)

formula is C3H6O3

11)

Consider the empirical formula,

C

H

O

2

(

M

W

e

=

45

g

mol

)

of a compound with

M

W

m

≈

135

g

mol

.

At this point, since this compound has an abundance of oxygen atoms, the most efficient method of solution (although not extremely versatile) is merely multiplying the empirical formula's weight by successive integers until we arrive at the molecular weight. It's as easy as,

1

M

W

e

=

45

g

,

2

M

W

e

=

90

g

3

M

W

e

=

135

g

I had to! Thus, the molecular formula is,

C

3

H

3

O

6