The empirical formula of a compound of molecular mass 120 is CH2O. What is the molecular formula of the compound?

The empirical formula of a compound is the lowest whole-number ratio of atoms in the compound. The molecular formula gives the actual number of each type of atom in a molecule. For example, the compound N_2O_2 has the empirical formula NO.
The molar mass of a compound is a whole-number multiple of its empirical formula mass. 
Let's start by finding the empirical formula mass of this compound:
CH_2O = C + (2)H + O = 12.0 + 2(1.0) + 16.0= 30 grams per mole
We can find the factor by which the molecular formula is larger than the empirical formula by comparing the two masses. Divide the actual molar mass of the compound by the empirical formula mass:
120/30 = 4
Since the molar mass of the compound is four times the empirical formula mass, the molecular formula is four times the empirical formula:
4(CH_2O) = C_4H_8O_2
The linked video provides a detailed explanation and some more examples of this process.