How many oxygen atoms are there in 1.55 ng of Ca3(PO4)2?

Question

Please list the steps you took to get this answer.

Answer 1

Ca3(PO4)2 has a molecular weight of 310. So 1 mole of this compound weighs 310 g.

1.55 ng/3.1E11ng = x moles

x = 5E-12 moles

since 1 mole = 6.023E23 molecules, 1.55 ng =3.0115E12 molecules.

each molecule has 8 oxygen atoms, so 1,55 ng has 24.1E12 atoms of oxygen

Answer 2

GMM of Ca3(PO4)2 = 120 + 2*(31+64) = 310 g

1.55 ng will be 5 x 10^-12 moles

So, no. of O atoms = 8 x 6.023 x 5 x 10^23 x 10^-12

= 2.4 x 10^13 atoms.