[adkinsjr]

Suppose I want to find the formula for calcium phosphate. I know that the calcium cation has a charge +2. The calcium

(btw, is there forum code for typing equations here?)

The formula could be:

CaPO_4 if the charge of the PO_4 anion is -2. Or it could be
Ca(PO_4)_2 if the charge is -1. How do I determine the charge of a polyatomic anion?

[Borek]

You have to know it, just like you know Ca²⁺.

Do you know formula of phosphoric acid?

[adkinsjr]

Yes, it's H_3PO_4.

I know that the charge of the cation by looking at the periodic table. I am trying to follow the system step by step in my text book, but it just seems like a great big mess to me. I can't distinguish what facts I can deduce from the rules, and the facts I just have to remember. It's very frustrating how compounds are named.

For example, if I want to find the formula for Molybdemum IV Phosphate, I know that the charge of the cation is +4 from the roman numerals. Now all I know is that the net charge of the anion(s) is -4. So is it impossible to deduce the charge, and therefore the number, of PO_4 anions in this compound?

I could guess the the charge of one PO_4 anion is -4 and just right MoPO_4. Or he formula could be Mo(PO_4)_2 if the charge is -2, it could be Mo_2(PO_4)_4 if the charge is -1, etc.

[Borek]

H₃PO₄ it is.

What makes acid an acid? Take a look at these:

HCl

H₂CO₃

H₃PO₄

Three acids, each with different charge on anion. Can you deduce why?

[adkinsjr]

The number of hydrogen atoms present corresponds to the charge of the anions?

I think I see where your going with this. These acids provide a sort of reference. If I know the formula H_3PO_4 then I can deduce the charge of phospates. I know that sulfuric acid has the formula H_2SO_4, so this means that the sulfate has a -2 charge. That's interesteing. thanks

BTW, is there a document with the latex commands on this forum?

[BetaAmyloid]

Yes - for latex.

Click Latex Click

 

[Borek]

The number of hydrogen atoms present corresponds to the charge of the anions?

In most cases. You will learn that is not always the truth (especially in organic compounds), but in the case of inorganic acid chances of making an error are very small.

I think I see where your going with this. These acids provide a sort of reference. If I know the formula H_3PO_4 then I can deduce the charge of phospates. I know that sulfuric acid has the formula H_2SO_4, so this means that the sulfate has a -2 charge. That's interesteing. thanks

Exactly, good to know it clicked

BTW, is there a document with the latex commands on this forum?

We don't use LaTeX for formulas - basically nothing wrong with it, but it is not necessary. Format your text with subscripts and superscripts -

Code: [Select]

PO[sub]4[/sub][sup]3-[/sup] gives PO₄^3-.