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Homework Help: Potassium Alum

  1. Sep 26, 2006 #1
    Hi guys,

    I know the formula for Potassium Alum is:

    [tex]2KAl(SO_{4})_{2} * 12H_{2}O[/tex]

    Now does this mean that there are 2 Moles of Potassium Alum or 24 moles of Potassium Alum if you include the 12 water molecules? I know when you calculate the molar mass of Potassium Alum, you need to add in the molar mass of the 12 water molecules.

    Thanks
     
  2. jcsd
  3. Sep 26, 2006 #2
    It means there are 2 moles of potassium alum in that formula as written. The '*' just means that each molecule set [tex]2KAl(SO_{4})_{2}[/tex] is surrounded by 12 molecules of water. But if that is the exact formula for potassium alum then [tex]2KAl(SO_{4})_{2} * 12H_{2}O[/tex] is only 1 mole. There are 2 moles of potassium and 2 moles of aluminum, but that entire structure is only 1 mole of potassium alum.
     
  4. Sep 26, 2006 #3
    Hi dmoravec, thanks for the reply. I am just confused on how there can be only 1 mole for that whole structure.

    I just need help on converted moles of aluminum to moles of alum.

    [tex]2K^+ + 2Al^+3 + 4SO_{4}^-2 + 12H_{2}O --> 2KAl(SO_{4})_{2} * H_{2}O [/tex]

    Now does this mean that 2 moles of Aluminum react with 2 moles of Potassium Alum or just 1 mole, as you mentioned above, of Postassium Alum?

    Thanks again
     
  5. Sep 26, 2006 #4
    well, its a bit nitpicky and just depends on naming but it could be either depending on how you define 1 molecule of potassium alum and how exactly the right side of the equation is written. If potassium alum is defined as [tex]2KAl(SO_{4})_{2} * 12H_{2}O[/tex] then it would be 2 moles of aluminum equating with 1 mole of potassium alum. If potassium alum is defined as [tex]KAl(SO_{4})_2*12H_{2}O[/tex] then it would be 2 moles of aluminum equating with 2 moles of potassium alum.

    (the difference lies in the formula above being [tex] (2KAl(SO_{4})_2*12H_{2}O)[/tex] or
    [tex] 2(KAl(SO_{4})_2*12H_{2}O)[/tex])
     
  6. Sep 26, 2006 #5
    Thanks dmoravec...the formula is actually written as the latter, with the 2 on the outside of the entire formula, so it must be 2 moles of aluminum reacting with 2 moles of potassium alum, in other words a 1:1 ratio. Thanks again, it was a big help
     
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