- titration question

Quote:

Originally Posted by escapeartist

I was solving a titration problem (titrating a solution of KHO with a solution of H2SO4).

And the final question is the number of molecules of water produced during the titration. The problem is that the neutralization net ionic equation is given by:

HO-(aq) + H+(aq) -> H2O(l) (1)

and that gives me a correct result since I have the number of moles of HO- (and H3O+).

However if I use the equation

H3O+(aq) + HO-(aq) -> 2H2O(l) (2)

I get the double number of molecules which is obvious since the stoichiometry of this equation is different.

My question is: when should I use equation 1 and when should I use equation 2? And why do chemists use both since it can be a source of confusion?

Thanks.

Well...the question is worded awkwardly, and one could argue that either answer is correct. The reason both are used is that (1) is simpler and applies even in non aqueous solvents, and (2) is more technically correct if the reaction is taking place in water.

Technically, H3O+ is not a water molecule; it's a hydronium ion. If an H+ gets removed from it, one could say that a water molecule was "produced", since it was not technically a water molecule before the reaction took place. This argument gives a valid reason why two water molecules were produced, but lots of chem teachers wouldn't accept this answer because that's not the simple model beginning chem students are generally taught.

Even though (2) is technically the correct equation in an aqueous solution, (1) describes the species of interest; a hydronium ion is really basically just a water molecule that's giving an H+ a ride until it gets reacted off. So, it's convenient just to leave off the water molecule from reactant and product side, even if it's in aqueous solution. In water, (1) might actually be technically wrong, but it's a technicality, and (1) is easier and more convenient.

As far as which one should you use...well, use (2) if you're emphasizing the fact that H+ doesn't exist in free form in aqeous solution, or if you want to be as correct and clear as possible, or if your teacher prefers it. However for convenience, or if your teacher assumes you'll use it, use (1). For the purposes of a class it really depends on what your teacher wants you to use. You should talk to her about it.