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Molar mass of Al2(SO4)3•6H2O = 2x27 + 3(32 + 4x16) + 6(2x1 + 16) = 450 g mol-1
No. of moles of Al2(SO4)3•6H2O = mass/(molar mass) = 67.5/450 = 0.15 mol

1. Each mol of Al2(SO4)3•6H2O contains 2 mol of Al3+ ions.
No. of moles of Al3+ ions = 2 x 0.15 = 0.3 mol

2. Each mol of Al2(SO4)3•6H2O contains 6 mol of H2O molecules.
No. of moles of H2O molecules = 6 x 0.15 = 0.9 mol

3. Each mol of Al2(SO4)3•6H2O contains 3 mol of S atoms.
No. of moles of S atoms = 3 x 0.15 = 0.45 mol

4. Each mol of Al2(SO4)3•6H2O contains 12 mol of 0 atoms.
No. of moles of 0 atoms = 12 x 0.15 = 1.8 mol

5. Each mol of Al2(SO4)3•6H2O contains 5 mol of ions (2 mol of Al3+ and 3 mol of SO42-).
No. of moles of ions = 5 x 0.15 = 0.75 mol

2007-01-02 12:32:23 補充：
Take L = 6.02 x 10^23 mol^-14. No. of O atoms = 1.8L = 1.08 x 10^255. No. of ions = 0.75L = 4.52 x 10^23

2007-01-02 12:34:20 補充：
Amendment of 4.4. No. of O atoms = 1.8L = 1.08 x 10^24

1) Molar mass of Al2(SO4)3．6H2O : 27X2+3(32+16X4)+6(1X2+16) = 450g/mol

No. of mol of Al2(SO4)3．6H2O : 67.5/450 = 0.15 mol

No. of mol of aluminium ions : 0.15mol X2 = 0.3 mol

2) No. of mol of water molecules : 0.15mol X 6 = 0.9 mol

3) No. of mol of sulphur atoms : 0.15 mol X 3 = 0.45 mol

No. of sulphur atoms : 0.45L = 2.709X(10^23)

4) No. of mol of oxygen atoms : 0.15 mol X 18 = 2.7 mol

No. of oxygen atoms : 2.7L = 1.63 X (10^24)

5) No. of mol of ions : 0.15 mol X 5 = 0.75 mol

No. of ions : 0.75L = 4.52 X (10^23)