# Write an equation to illustrate that a reaction is acidic.

## Homework Statement

Write equations to illustrate whether P4O10(s) is an Arrhenius acid or a base.

## Homework Equations

P4O10(s) + H2O(l) will eventually have the H3O+ and a negative ion because it is a non-metallic oxide in water.

## The Attempt at a Solution

I can think of at least 10 different ways to come up with a H3O+ in the equation, but the answer has a specific answer which is,

P4O10(s) + 6H2O → 4H3PO4(aq)

I don't know why they have 6 H2O's.

I also don't know why they only use one of the 4 H3PO4(aq) for this equation,

H3PO4(aq) + H2O(l) → H3O+(aq) + H2PO4-(aq) .

I can't find anywhere in the chapter that explains how I would know this.

Bystander
Homework Helper
Gold Member
P4O10(s) + 6H2O → 4H3PO4(aq)
I don't know why they have 6 H2O's.
Try balancing the equation.
I also don't know why they only use one of the 4 H3PO4(aq) for this equation,
H3PO4(aq) + H2O(l) → H3O+(aq) + H2PO4-(aq) .
Did you wish to count individual molecules, or the stoichiometry?

• student34
epenguin
Homework Helper
Gold Member
For the first question you might find it easier to write it as P2O5, write an obvious covalent structure for it with pentavalent P. See how H2O can be added to that in two stages.

For your second question, the means to answer it may be in another chapter - on buffers, pH calculations etc. All three protons of H3PO4 can diissociate. But only the first one is strongly acid. Actually what they write seems to me slightly arbitrary. From the first Ka of around 0.01 M it follows (easily ) that if you dissolved the pentoxide to around 0.01 M [P] about half of it would be in form H2PO4-; the greater the dilution the greater this fraction, while at higher concentrations the majority would be undissociated H3PO4 - e.g. at 1M about 90% is, and only 10% is H2PO4-, and very little other ionised forms.

Rereading maybe that was not your question. The scheme is not meant to represent 'only one' ot the four molecules - it represents any of them equally. That reaction is extremely fast and goes back and forth something like 108 times per second.

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• student34
Try balancing the equation.

I understand that the equation is balanced. I just have no idea why they decided to end up with 4H3PO4(aq) instead of anything else. Of all of the possible molecules, how would I know to try using that molecule?

Did you wish to count individual molecules, or the stoichiometry?

This is half way through grade 11 chemistry (old student who is upgrading). I think stoichiometry is in the next chapter. So I think individual molecules would be expected for this question.

Borek
Mentor
I just have no idea why they decided to end up with 4H3PO4(aq)

First - P4O10 is an acid anhydride and is known to react with water producing phosphoric acid. My bet is you were told that earlier.

Second - phosphoric acid is an obvious example of an acid containing P, and being able of producing H3O+.

• student34