Test Question?

Question

So I just finished a chemistry test today and I know I answered this question incorrectly. Could anyone show me the proper way of doing it?

Q. How many atoms of oxygen are in 198.00 g of Ca3(PO4)2?

I only found the number of atoms in total and did nothing to find the number of oxygen atoms. Any help would be great!

Answer 1

1 mol of Ca3(PO4)2 contains 8 mol of Oxygen atoms. Thus, if you know the number of moles of Ca3(PO4)2 all you need to do is multiply by 8 to get the mols of oxygen atoms. To find the mols of the salt you need the molecular weight:

3 x 40 = 120 (Ca)
2 x 31.5 = 63 (P)
8 x 16 = 128 (O)

Total is: 120 + 63 + 128 = 311 g / mol

So, 1mol of Ca3(PO4)2 is the same as 311 g.

Mols of Ca3(PO4)2= 198 g / 311 g/mol = 0.6367 mol of Ca3(PO4)2

Mols of O atoms= 0.6367 mol x ( 8 mol O atoms / 1 mol Ca3(PO4)2)= 5.0932 mol of O atoms

Since 1 mol = 6.02x10^23 atoms we have:

5.0932 mol O atoms x 6.02x10^23 O atoms/ mol = 3.0661 x 10^24 atoms of O in 198 g of Ca3(PO4)2

Answer 2

well, if you found the total number of atoms, then the total number of atoms of oxygen is that number times 8/13. If instead you mean that you found the total number of molecules in that sample, then you need to multiply that number by 8 to get atoms of oxygen

Answer 3

well there are 8 atoms of O in one molecule
one mole of that compound weighs 310 grams
198/310 = .6387096774, meaning there are that many moles of the molecule
one mole is 6.02E23, so if u have 3.845032258E23 molecules(1 mole times the number of moles)
for every molecule u have 8 oxygen
so u get
3.076025806E24 atoms of oxygen
or approximately
3076025806000000000000000 atoms