What is the mass of one mole of atoms in these 3 compounds?

For Li₂PO₄ :
Mass of 1 mole of Li₂PO₄ = (6.9×2 + 31.0 + 16.0×4) g = 108.8 g
1 mole of Li₂PO₄ contains 7 moles of atoms (2 mol Li + 1 mol P + 4 mol O).
Mass of 1 mole of atoms = (108.8 g) / 7 = 15.5 g

However, Li₂PO₄ does not exist. It should be Li₃PO₄. For Li₃PO₄ :
Mass of 1 mole of Li₃PO₄ = (6.9×3 + 31.0 + 16.0×4) g = 115.7 g
1 mole of Li₃PO₄ contains 8 moles of atoms (3 mol Li + 1 mol P + 4 mol O).
Mass of 1 mole of atoms = (115.7g) / 8 = 14.5 g


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For H₄Mg₃Si₂O₉ :
Mass of 1 mole of H₄Mg₃Si₂O₉ = (1.0×4 + 24.3×3 + 28.1×2 + 16.0×9) g = 277.1 g
1 mole of H₄Mg₃Si₂O₉ contains 18 moles of atoms (4 mol H + 3 mol Mg + 2 mol Si + 9 mol O).
Mass of 1 mole of atoms = (277.1 g) / 18 = 15.4 g


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For Ca(C₂H₃O₂)₂ :
Mass of 1 mole of Ca(C₂H₃O₂)₂ = (40.1 + 12.0×4 + 1.0×6 + 16.0×4) g = 158.1 g
1 mole of Ca(C₂H₃O₂)₂ contains 15 moles of atoms (1 mol Ca + 4 mol C + 6 mol H + 4 mol O).
Mass of 1 mole of atoms = (158.1g) / 15 = 10.5 g

Li2PO4
Molar mass = 115.7944g/mol
115.7944g/mol divide by 6.022 x 10^23 mol^-1 = 1.922 x 10^-22

showed you one now you can do the rest