# Potassium Alum

1. Sep 26, 2006

### skybox

Hi guys,

I know the formula for Potassium Alum is:

$$2KAl(SO_{4})_{2} * 12H_{2}O$$

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. Sep 26, 2006

### dmoravec

It means there are 2 moles of potassium alum in that formula as written. The '*' just means that each molecule set $$2KAl(SO_{4})_{2}$$ is surrounded by 12 molecules of water. But if that is the exact formula for potassium alum then $$2KAl(SO_{4})_{2} * 12H_{2}O$$ 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.

3. Sep 26, 2006

### skybox

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.

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

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

4. Sep 26, 2006

### dmoravec

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 $$2KAl(SO_{4})_{2} * 12H_{2}O$$ then it would be 2 moles of aluminum equating with 1 mole of potassium alum. If potassium alum is defined as $$KAl(SO_{4})_2*12H_{2}O$$ then it would be 2 moles of aluminum equating with 2 moles of potassium alum.

(the difference lies in the formula above being $$(2KAl(SO_{4})_2*12H_{2}O)$$ or
$$2(KAl(SO_{4})_2*12H_{2}O)$$)

5. Sep 26, 2006

### skybox

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