Question

What is the molecular formula of a compound with a mass of 180 grams and an empirical formula of `CH_2O`?

Answer 1

`"C"_6"H"_12"O"_6`

Explanation:

First, calculate the formula mass of the empirical formula:

`"M"_r` (`"CH"_2"O"`)`= 12 + 2xx1 + 16 = 30`

Next we need to find what I personally refer to as the multiplier: it is not given an official name, but it is the number that we will multiply the mole ratios (the subscript numbers) in the empirical formula by to give the molecular formula. Observe this simple equation:

`"multiplier"` `= ("M"_r("molecular formula"))/("M"_r("CH"_2"O"))`

`=> "multiplier"` `= 180/30 = 6`

Now, put that multiplier into effect:

`"molecular formula"` `= "C"_ (1xx6) "H"_ (2xx6) "O"_ (1xx6) = "C"_6"H"_12"O"_6`.