Chemistry Problem: atoms and molecules?

Question

Hi I am really stuck on these questions and any help would be appreciated:

1. calculate the number of atoms present in 1.50g of silver (Ag)

2. calculate the number of molecules present in 4.75 millimol of phosphine (PH3)

3. calculate the number of moles of the positive ions present in 9.71 grams of barium phosphide (Ba3P2)

Answer 1

1. Things to help you: Molar mass of Silver = 107.8682 g/mol
Avogadro's number = 6.02x10^23 atoms Ag/ mol Ag

1.50g Ag / 108 g/mol Ag = 0.0139 mol Ag
0.0139 mol Ag * 6.02x10^23 atoms Ag/mol Ag = 8.36x10^21 atoms Ag

2. 1000 millimol = 1 mol
Avogadro's number = 6.02x10^23 molecules PH3 /mol PH3

4.75 mmol = 4.75 x 10^-3 mol
4.75x10^-3 mol * 6.02x10^23 molecules/mol = 2.86x10^19 molecules PH3

3. Molar mass Ba = 137.327 g/mol
Molar mass P = 30.973762 g/mol
3 Ba ions per molecule of Ba3P2

Molar mass Ba3P2 = 137.327 x 3 + 31 x 2 = 474 g/mol

9.71 g / 474 g/mol = 0.0205 mol Ba3P2

0.0205 mol Ba3P2 * 3 mol Ba ions/mol Ba3P2 = 0.0615 mol Ba ions