Question

How many atoms of phosphorus are in 8.10 mol of copper(II) phosphate?

Answer 1

You know from the chemical formula that 1 mole of Cu3(PO4)2 contains 2 moles of P so you can work out how many moles of P are in 8.1 moles of Cu3(PO4)2. 

As for anything, 1 mole of X contains 6.022 * 10^23 of X. Multiply moles of X by Avogadro's number.

Answer 2

Similar to what 'Rosael' noted (thanks Rosael!) instead of just multiplying the 2 mols that Phosphorus has, in it's Cu3(PO4)2 compound... we need to recognize that the total amount of moles in 8.10 moles of the compound is actually (2 mols P x 8.10 mols of Cu3(PO4)3).... from here we know that Cu3(PO4)2 contains 16.2 mols of P.... but because we are asked to figure out how many ATOMS 16.2 MOLS of P actually are... this is where we utilize Avagadro's wonderful #.... (i saw wonderful because it magically helps us switch back and forth from these sorts of things.)

We do this by saying:

16.2 mols of P x ( 6.022x10^23 atoms of P ) / 1 mol of P

writing this out will help you to see how the mols then cancel, and we end up only with atoms of P.... *Rember that Avagadro's # is there to help! It may not always go on top... sometimes you will need to divide by it to find how many mols is in something...* Hope this helped!