Finding Empirical and molecular formulas using molar mass?

Question

Carbohydrates are compounds containing carbon, hydrogen, and oxygen in which the hydrogen to oxygen ratio is 2:1. A certain carbohydrate contains 40.0 percent carbon by mass. Calculate the empirical and molecular formulas of the compound if the approximate molar mass is 178 g.

Answer 1

If the molar mass is 178 g and the substance is 40% carbon by mass, the molar mass of the carbon in the substance is 178*0.4 = 71.2. The molar mass of a carbon atom is 12, and 71.2 / 12 = 5.93, so we can safely assume there are 6 carbon atoms in the compound. The remaining molar mass is 178 - 71.2 = 106.8 g. A single unit of H2O has a molar mass of 1*2 + 16 = 18, and 106.8 / 18 = 5.9333333333, so we can again say that there are 6 such units. That means we have C6H12O6, the molecular formula. The empirical formula requires division by the greatest common factor, in this case 6, leaving us with CH2O.

Answer 2

Perhaps if we knew what your empirical formula is and how you arrived at it. That would be helpful. Clearly, the empirical formula cannot be greater than the molecular formula.

Answer 3

David is absolutely right - no other answer.

Extra thoughts:
I seriously think there is a little mistake in the question. The molecular weight should have been 180.

Guess your word "approximate" will save your question.