# How many moles of phosphorous do I have if its mass is 17g? We are talking about phosphorus and converting grams of it into moles.  Pure phosphorus is generally found in two forms, red and black.  Both have a molecular formula of P as it is found on the periodic table.  The key to answering this question is to use the atomic masses as found on the periodic table.  The atomic mass is basically the weight in grams that 1 mole of that element would weigh (it is technically an average mass of all the isotopes of that element in their naturally occurring proportions but that is beyond what you are looking for I'm sure).  So basically, the atomic mass has units of "grams per mole."  In the case of phosphorus, the atomic mass as found on any periodic table is 30.973762 grams per mole.  We have 17 grams of P, so we divide 17 by the atomic mass to get the amount of P in moles.  The equation is below:

17 grams / (30.973762 grams/mole) = 0.5489 moles

So 17 grams of phosphorus is equal to 0.5489 moles of phosphorus

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