How would you find the empirical formula given a particular molar mass?

Take, for example, benzene, C6H6, molecular weight 78. The empirical formula is CH. The formula weight of CH is 13.

So: You would have to consider the formula weight of each empirical formula among the answers, Then you would have to multiply each formula weight by various numbers to see whether a product would give a fit with "x g/mole"

For the question you gave, as they list out all the possible formulas, you can calculate the molar mass of each and choose the one that matches the molar mass they gave.

On non-multiple choice questions however, you need to know the percent composition of each element in the compound in order to determine an empirical formula with the molar mass. In such a case, you can assume a total mass of the compound. For e.g., if the question is:

"A compound is 48.8% carbon, 5.05% hydrogen, 14.14% nitrogen, 32.32% oxygen, what is its empirical formula?"

In such an e.g., you can assume 100 g of the compound...so that gives you C: 48.8g, H: 5.05g, N: 14.14g, O: 32.32 g. As you now know the mass of each element, and you already know their molar masses,  you can calculate the number of moles of each element.

In the question I gave, there is 4.063 moles of C, 5 moles of H, 1.01 moles of N and 2.02 moles of O.

Divide these values by the lowest value (1.01) (to get the lowest, whole number ratio), and with a little bit of rounding you get: C 4 H 5 N 1 O 2 for the empirical formula. Sometimes, after dividing, you don't get such clear cut ratios...in these cases, you have to multiply by w/e value to make all the numbers whole.

My teacher has this rhyme for calculating empirical formulas:

Assume a mass,
Calculate the mole,
Divide by lowest,
Multiply till whole.

It summarizes what I said pretty well...and it rhymes!

Hope that helps. And GL on your exam! (mine is tomorrow )