# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: JP_SKYS on September 26, 2010, 08:01:32 PM

Title: Finding the # of atoms in one formula unit.
Post by: JP_SKYS on September 26, 2010, 08:01:32 PM
How many atoms (total) are in one formula unit of Ca3(PO4)2?  (I know that the answer is 13 b/c of the answer key, but I am having trouble doing the actual work and I come no where close to 13.)

My attempt:

The total grams of the above is 310.18 grams.  I found the % of each of the elements by dividing the total grams of each element by the total grams (310.18) x 100 and got the following: (Ca - 39%), (P - 20%) and (0 - 41%).
I then found the amount of moles of each element by dividing its percent by its atomic mass and I got the following:  (Ca - .9730538922), (P - .6457862448) and (O - 2.5625).
I then divided the amount of moles that I got for each element by the smallest one (the moles of phosphorus) to get the empirical formula and came up with Ca2PO4

For my conversion from grams to atoms I calculated the number of grams in the empirical formula and came up with 175.13 g.  I converted this to atoms by dividing this number by 310.18g (the original number of grams in the original molecular formula given) and multiplied it by 6.02x1023 and I am getting 3.4x1023.

I have looked it over a few times and re-done the calculations and not sure if I am missing a step, doing a step completely wrong ...  ???
Title: Re: Finding the # of atoms in one formula unit.
Post by: V on September 26, 2010, 10:14:19 PM
You are making it a lot more difficult than it needs to be. If you count how many atoms are written in Ca3(PO4)2 what do you come up with? How many Ca atoms are there? How many O? How many P?
Title: Re: Finding the # of atoms in one formula unit.
Post by: JP_SKYS on September 27, 2010, 12:56:15 AM
You are so right. There are 3 Ca, 2 P and 8 O atoms.  If I divide each of them by 2 I cut out all the extra work that I did in the beginning.  Thank you! But I still can't make it to the answer - 13.
Title: Re: Finding the # of atoms in one formula unit.
Post by: sjb on September 27, 2010, 02:46:58 AM
You are so right. There are 3 Ca, 2 P and 8 O atoms.  If I divide each of them by 2 I cut out all the extra work that I did in the beginning.  Thank you! But I still can't make it to the answer - 13.

If there are 3 calciums, 2 phosphorouses, and 8 oxygens in one formula unit, how many atoms all together?
Title: Re: Finding the # of atoms in one formula unit.
Post by: JP_SKYS on September 27, 2010, 10:37:59 AM
ok I am an idiot. thank you     :-\
Title: Re: Finding the # of atoms in one formula unit.
Post by: sjb on September 27, 2010, 12:34:15 PM
ok I am an idiot. thank you     :-\

Don't be too hard on yourself, sometimes we all miss things. Even great mathematicians can have difficulty with basic arithmetic.