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
#176.1cancel("g") * "40.92 g C"/(100cancel("g")) = "72.06 g C"#
#176.1cancel("g") * "4.58 g H"/(100cancel("g")) = "8.065 g H"#
#176.1cancel("g") * "54.50 g"/(100cancel("g")) = "95.97 g O"#
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
#"For C": (72.06cancel("g"))/(12.0cancel("g")/"mol") = 6.005 ~= 6#
#"For H": (8.065cancel("g"))/(1.01cancel("g")/"mol") = 7.985 ~= 8#
#"For O": (95.97cancel("g"))/(16.0cancel("g")/"mol") = 5.998 ~= 6#
Therefore, the molecular formula of ascorbic acid is
#C_6H_8O_6#
One mole of ascorbic acid contains 6 moles of carbon, 8 moles of hydrogen, and 6 moles of oxygen.
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Rowan C.
asked • 02/01/18
vitamin C (absorbic acid) contains 40.92% C, 4.58%H , and 54.50 O ?? what is the empirical formula for vitamin C
1 Expert Answer
When given % composition, and asked for the empirical formula, it is easiest to just assume 100 g of material. Thus, Mass C = 40.92 g. Moles C = 40.92 g x 1 mole/12 g = 3.41 moles C
Mass H = 4.58 g. Moles H = 4.58 g x 1 mole/1.0 g = 4.58 moles H
Mass O = 54.50 g. Moles O = 54.50 g x 1 mole/16 g = 3.41 moles O
Next, we want to get the moles into whole numbers, so we begin by dividing all by the smallest, i.e. divide all by 3.41.
Moles H = 4.58/3.41 = 1.34
Now, in order to get 1.34 to be a whole number we multiply it (and all others) by 3
Empirical Formula = C3H4O3