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Question

In 67.5g of hydrated aluminium sulphate Al2(SO4)3．6H2O ,calculate
1)the number of moles of aluminium ions

2)the number of moles of water molecules

3)the number of sulphur atoms

4)the number of oxygenatoms

5)the number of ions
(Relative atomic masses:H=1;O=16;Al=27;S=32)

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Answer 1

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

Answer 2

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)