Problem: Calculate the molarity of 11.2 grams of CaCO3 in 120mL of solution.
I do know to start like this 11.2g(1mol)/_____g=_____ mol but I am not sure what to divide 11.2g(1 mol) by in order to get the moles

CaCO_3 is calcium carbonate.

Every pure substance has a "molecular weight". The molecular weight is found by adding the atomic weights of the three atomic components

Ca - 40.078
C - 12.0107
O - 15.9994 * 3 = 47.9982

Add them up and get 100.0869 (grams/mole)

If you divide grams by grams/mole

\frac{grams}{[\frac{grams}{mole}]} = mole

So, divide the weight of material by the molecular weight and get the number of moles.

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If you have moles and want to convert to grams, simply multiply by the molecular weight instead.

moles [{\frac{grams}{mole}] = grams