potassium aluminium sulphate

As you know K+ ; Al +3 ; SO4 -2
For K and Al, we get +4
For (SO4)2, we get 2*-2 = -4
KAl(SO4)2 is netural (+4-4 = 0) and thus, becomes the empirical formula of potassium aluminium sulphate.

2009-01-09 08:44:15 補充：
Na+ ; H+ ; P +5 ; O -2
Na2HPO4: 2*(+1)+(+1)+(+5)-4*(-2) = 8-8 = 0

2009-01-09 15:00:54 補充：
potassium: K (+)
aluminium: Al (3+)
sulphate: SO4 (2-)
By balancing the electrons(or charges), you will get potassium aluminium sulphate = KAI(SO4)2

sodium: Na(+)
monohydrogen (one hydrogen): H(+)
phosphate: PO4 (3-)
Since the number of hydrogen is fixed, you can only raise the Na ratio.

The compound potassium aluminium sulphate contains two cations (K+ and Al3+) and one anion (SO42-), and this is known as a double salt. A double salt is a salt that contains two more than one cation or anion.

Aluminium sulphate usually exists as a hydrated salt, and its empirical formula can be written either as K2SO4•Al2(SO4)3•24H2O or KAl(SO4)2•12H2O. The former is easier to understand, as it shows that the double salt contains the structure of K2SO4 (K+ is monovalent and SO42- is divalent) and Al2(SO4)3 (Al3+ is trivalent and SO42- is divalent), and both of them are known. However, the latter is simpler and thus is usually used.

Sodium monosodium phosphate is a simple salt, which contains Na+ cation and HPO42- anion. Since Na+ is monovalent while HPO42- is divalent, the empirical formula is thus written as Na2HPO4.
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