Cemistry HELP!

[SNF]

Hello there,

Our chem teacher told us to solve this question; An organic compound contains 58.8% Carbon, 9.8% Hydorgen and 31.4% oxygen by mass. Its moral mass is 102 g mol-

a) calculate the empirical formulae

b) calculate the molecular formulae

It doesn't make any sense to me cuz when I work it out I get C5H10O4. The mass of this compound is 102 so does that mean that the molecular formulae is the same as the empirical formulae

[FChaosi_]

Alright, so it's assumed we're using 100 grams (x% = x grams).
So we have 58.8 grams of C, 9.8 grams of H, and 31.4 grams of O.
We can't determine the empirical formula by using only the weights as the weights don't tell us the number of atoms of each element.

Number of moles of carbon = m/M
Number of moles of carbon = 58.8 g/12.01 g mol^-1 
Number of moles of carbon = 4.90 mol (3sf)

Number of moles of hydrogen = m/M
Number of moles of hydrogen = 9.8 g/1.01 g mol^-1
Number of moles of carbon = 9.7 mol (2sf)

Number of moles of oxygen = m/M

Number of moles of oxygen = 31.4 g/16.00 g mol^-1
Number of moles of oxygen = 1.96 mol (3sf)

Therefore the approximate ratios are:
C:H:O = 5:10:2
 

Thus the empirical formula is C₅H₁₀O₂.

To find the molecular formula, we need to know "how many empirical formulae make up how many molecular formulae".

Empirical formula mass = 5(12.01 g mol^-1) + 10(1.01 g mol^-1) + 2(16.00 g mol^-1)
Empirical formula mass = 102.15 g mol^-1

This is close enough to the molecular mass, 102 g mol^-1, for us to assume that the empirical formula is the molecular formula.
So the molecular formula is also C₅H₁₀O_2.

Edited September 18, 2015 by FChaosi_