How many moles of phosphorous do I have if its mass is 17g?

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ncchemist eNotes educator| Certified Educator

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

simonetaylor2 | Student

We can start out with knowing some of the basic stats about phosphorus. It's important to know the atomic weight of an atom before you can determine how many moles of it you have. The atomic weight of phosphorous is 30.97 grams. The atomic number of any element is what one mole of its atoms would weigh in grams. A mole is equal to 6.02 x 10^23 atoms. So, in the case of phosphorus, one mole of it would be equal to 30.97 grams. In order to find the amount of moles in 17 grams of phosphorus, we can divide the amount we have by the atomic weight. (It's sort of like finding the percentage of something by doing the part over the whole.)

17g/30.97g = 0.549

So there are 0.549 moles of phosphorus in 17 grams.

mere310ann | Student

Converting elements from grams --> moles OR moles --> grams is a common step in many stoichiometry problems.

First, it's important to identify the atomic mass of the element(s) involved, which in this case is phosphorus.  Find phosphorus on the periodic's in the 3rd row/15th column & is symbolized by "P."  If you look more closely, you'll see various numbers in the box...the numbers in the top right corner specify the element's electron configuration, the number in the top left is the atomic # (which identifies the element by its number of protons), and the number at the bottom is the atomic mass.

The atomic mass tells us that 1 atom of phosphorus weighs approximately 30.97 amu (atomic mass units).  Similarly, this number tells us that 1 mole of phosphorus weighs approximately 30.97 g (grams).

*NOTE: There are 6.022×1023 phosphorus atoms in 1 mole of phosphorus...that's a lot of atoms!  And this is why we can weigh moles in grams!

So the atomic mass tells us that 1 mole phosphorus = 30.97 grams phosphorus.  Soooo...what now???  We can use this as a conversion factor to help answer your question!

Let's go back to the question for a minute...what number/unit are you starting with? 17 g phosphorus!  I suggest writing that value as a fraction (which in this case we can do by placing the whole thing over a denominator of 1).

Next question...what are you trying to find?  Number of moles of phosphorus!  So we need to change our unit from grams --> moles, and we can use our conversion factor to do this.  

We need to write our conversion factor (1 mole phosphorus = 30.97 grams phosphorus) as a fraction, but what do we put in the numerator & what do we put in the denominator??  Since we want to change from grams --> moles, we need to get rid of the "grams" unit in our 1st fraction.  We can achieve this by placing the "grams" unit in the opposite spot in our 2nd fraction...since 17 grams were in the numerator of the 1st fraction, we put 30.97 grams in the denominator of the 2nd fraction.  Consequently, 1 mole will go in the numerator of the 2nd fraction.

All that remains is multiplying the two fractions together...multiple the numerators together, multiply the denominators together, and simplify!  The "grams" should cancel each other out, and you should be left with an answer of 0.5489 moles of phosphorus.


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