What is the molecular formula of the alcohol?

Question

An alcohol is 64.81% C, 13.60% H, and 21.59%. O by mass. Another experiment shows that its l11ol~cu'ar weight is approximately 74 amu. What is the molecular formula of the alcohol?

Answer 1

To solve these, first divide the weight % by the atomic weight to get relative amounts by numbers of atoms.

64.81/12.01 = 5.396
13.60/1.01 = 13.46
21.59/16.00 = 1.349

These values are proportional to the formula, but we need to get this into whole numbers. Let's try dividing them by the smallest number (its a good staring point).

5.396/1.349 = 4.00
13.46/1.349 = 9.978
1.349/1.349 = 1

These valuse are close enough to whole numbers to be OK; now, we have what is called an empirical formula: C4H10O. This may, or may not be the actual formula. The actual formula could be a multiple of this. Now, let's see what the empirical formula weighs:

(12.01 x 4) + (1.01 x 10) + 16.00 = 74.14g

Since the molecular weight was already determined to be ~74 the actual formula is the same as the empirical formula: C4H10O

Answer 2

Assume 100.0g of alcohol.
Find number of moles of each element by dividing by molar mass:
C: 64.81g/12.01g/mol = 5.396 mol C

H: 13.60g/1.008g/mol = 13.49 mol H

O: 21.59g/16.00g/mol = 1.349 mol O

Divide all moles of each element by the lowest number of moles, to get smallest whole number ratio of each element:

C: 5.396/1.349 = 4.000

H: 13.49/1.349 = 10.00

O: 1.349/1.349 = 1.000

Empirical formula: C4H10O

Formula weight of C4H10O
C:....4(12.01) = 48.04
H: 10( 1.008) = 10.08
O: ..1(16.00) = 16.00
Total FW =.......74.12

Since there is only 1 FW in the molecule 74/74.12 ~ 1
the molecular formula of the alcohol is C4H10O.

If there had been 2 FW in the molecule, i.e., amu had been 148, then multiply all the subscripts by 2: C8H20O2.

Answer 3

calculate the relative number of moles of C, H, and O-
C: 64.81/12.011 = 5.395
H: 13.60/1.008 = 13.492
O: 21.59/16.00 = 1.349
divide each by 1.349
C: 5.395/1.349 = 4.00
H: 13.492/1.349 = 10.00
O: 1.349/1.349 = 1.00
empirical formula is C4 H10 O
that is also the molecular formula (molar mass = 74.1)