Molecular (True) Formula?

Question

How do you solve a molecular formula? Can someone describe how to do it and do the first problem for me while explaining the steps as you go along?
Thanks.

A compund has the following percentage composition: 26.7% Carbon: 2.2% hydrogen; 71.1% oxygen. The molecular weight of this compound is 90. What is the compound's true formula?

Answer 1

The data is given in % by weight but you need to know in terms of numbers of atoms to get a formula, so you need to take each weight % and divide by it's respective molar mass:

26.7%C/12.01 = 2.223
2.2%H/1.01= 2.18
71.1%O/16.00 = 4.44

These numbers are now proportional to the emperical formula, but we need to get them into whole numbers. First, we'll try dividing all of them by the smallest number. (Note: sometimes you need to try multiplying or dividing a couple of times to get all them to be whole numbers). In this case, it only took one try.

2.223/2.18 = 1.02
2.18/2.18 = 1.00
4.44/2.18 = 2.04

They all came out nearly whole numbers,so the emperical formula is CHO2 which weighs 45.02g

Now, we need to see how many of the empirical formulas fit into the stated molecular weight:

90/45.02 = 1.999 So the formula is C2H2O4

Answer 2

first you convert percentages into grams (26.7%=26.7 g)

so
c= 26.7 g
h= 2.2 g
o= 71.1 g

then convert to moles
so
c= 2.22
h= 2.18
o= 4.4 mol

then divide each amount by the smallest (which is 2.18 mol) to come up with a mole ratio

so for..
c= [2.22/2.18]= 1.01 which is about equal to 1
h= [2.18/2.18] = 1
o= [4.4/2.18] = 2.01 which is about equal to 1

so your empirical formula is CHO2

now for the molecular formula, it's molar mass\formula mass

so 12.01+1.008+(16*2)= 45.018 is your formula mass

if the molar mass is 90(given), 90/45=2

so your answer would be C2H2O4