Question

What is the molecular formula for a compound if it is 39.99% carbon, 6.73% hydrogen, and 53.28% oxygen, and has a molar mass of 180.18 g/mol?

Answer 1

To find the molecular formula of a compound, convert the percent composition to grams for a 100 g sample, determine the mole ratio for each element, use this to calculate the empirical formula, then adjust it according to the molar mass of the compound.

Step-by-step explanation:

To determine the molecular formula for a compound given its percent composition and molar mass, we first calculate the empirical formula from the percent composition, then use the molar mass to find the ratio between the empirical formula's mass and the molar mass to determine the true molecular formula.

The molar masses of carbon, hydrogen, and oxygen are 12.01 g/mol, 1.008 g/mol, and 16.00 g/mol respectively. If a compound is 39.99% carbon, 6.73% hydrogen, and 53.28% oxygen with a molar mass of 180.18 g/mol, we start by converting the percentages to grams assuming a 100 g sample; then we divide by the respective atomic weights to get the ratio of moles of each element, simplifying to the smallest whole number ratio for the empirical formula, which gives us CxHyOz.

The next step is dividing the molar mass of the compound by the molar mass of the empirical formula (CxHyOz) to determine the multiple that will be used to convert the empirical formula into the molecular formula. Finally, we multiply the subscripts of the empirical formula by this multiple to find the molecular formula.

Answer 2

If molar mass of compound is 180.18 g/mol and of that mass 39.99 % is Carbon
∴ molar mass of carbon in compound = 180.18 g/mol × (39.99 ÷ 100)
= 72 g/mol
since a single carbon = 12 g/mol
then number of carbons in compound = 72 ÷ 12 g/mol
= 6 CARBON atoms
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If molar mass of compound is 180.18 g/mol and of that mass 6.73 % is hydrogen
∴ molar mass of hydrogen in compound = 180.18 g/mol × (6.73 ÷ 100)
≈ 12 g/mol
since a single hydrogen = 1 g/mol
then number of hydrogen in compound = 12 ÷ 1 g/mol
= 12 HYDROGEN atoms
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If molar mass of compound is 180.18 g/mol and of that mass 53.28 % is Oxygen
∴ molar mass of oxygen in compound = 180.18 g/mol × (53.28 ÷ 100)
≈ 96 g/mol
since a single oxygen= 16 g/mol
then number of oxygen in compound = 96 ÷ 16 g/mol
= 6 OXYGEN atoms

∴ molecular formula of compound (aldehexose sugar e.g. glucose) = C₆H₁₂O₆