how do you convert from mass to moles?

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You must first find the molar mass of the element or compound (use the periodic table). If the chemical is an element, just read off the atomic mass from the periodic table. If it is a compound, you must know the molecular formula, and then you find the total molar mass of the compound by adding up the atomic masses of each atom in the compound. The unit of the molar mass will be in grams per moles (g/mole). Once you have the molar mass, you can easily convert from grams to moles, and also from moles to grams. Number of moles = (# of grams) ÷ (molar mass)Number of grams = (# of moles) × (molar mass)Here is an example: How many moles are in 5 grams of O2? The molar mass of O2 = 16.00 g/mole x 2 (for 2 atoms of oxygen) or 32.00 g/mole. 5 grams of O2 ÷ (32 g/mole) = 0.15625 moles 

It is quite simple.   The number of moles will equal to the weight, divided by the atomic mass (Usually multiplied by 2, becuase of bonding) Like O2, F2, Cl2 etc. So, 1 gram of fluorine will equal to (19x2) (F2), which will be 38.   Then, 1 (gram) / 38 = 0.026315 moles of fluorine