The molecular formula is
We need to determine the empirical formula and the empirical formula mass. Then divide the molecular mass by the empirical mass, then multiply the empirical formula times the result.
Determine the masses
The percentages given add up to
Determine the molar masses of the elements (atomic weight in g/mol).
Determine the number of moles of each element from the given mass and the molar mass.
Determine mole ratios by dividing the moles for each element by the smallest number of moles.
Multiply the ratios times
The empirical formula is
The empirical formula mass =
Divide molecular mass by empirical mass. Multiply the empirical formula times the result.
Here's an alternative technique you can use to get the molecular formula.
If you know the percent composition and the molar mass of a compound, here's how you can determine its molecular formula.
A compound's molar mass tells you what the exact mass of 1 mole of that compound is. In your case, you know that ascorbic acid has a molar mass of 176.1 g/mol.
This means that 1 mole of ascorbic acid has a total mass of 176.1 g. Let's assume that you have a 176.1-g sample of ascorbic acid. According to ascorbic acid's percent composition, every 100 g of acid will contain 40.92 g carbon, 4.58 g hydrogen, and 54.50 g oxygen.
This means that your 176.1-g sample will contain
To get the number of moles each element contributes to 1 mole of ascorbic acid, simply divide these numbers by each element's molar mass
Therefore, the molecular formula of ascorbic acid is
One mole of ascorbic acid contains 6 moles of carbon, 8 moles of hydrogen, and 6 moles of oxygen.