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

How many atoms (total) are in one formula unit of Ca_{3}(PO_{4})_{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 Ca_{2}PO_{4}.
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 6.02x10^{23} and I am getting 3.4x10^{23}.
I have looked it over a few times and redone the calculations and not sure if I am missing a step, doing a step completely wrong ... ???

You are making it a lot more difficult than it needs to be. If you count how many atoms are written in Ca_{3}(PO_{4})_{2} what do you come up with? How many Ca atoms are there? How many O? How many P?

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.

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?

ok I am an idiot. thank you :\

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.