# MCAT 5R questions

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#### OnMyWayThere

##### OMS-III
15+ Year Member
20+ Year Member
On #23, it states: In the equilibrium constant expression for 2(NH4)2HPO4(s) + H2O(l) ---> 4NH4+(aq) + HPO4-(aq) + H2PO4-(aq) + OH-(aq),..... [H2O] is omitted because the salt is:

A. only weakly basic, and [H2O] is nearly constant

C. only weakly acidic, and [H2O] is nearly constant

So, how does one choose whether the water was acidic or basic in the reactants?

Thanks... I might post some more questions if I don't understand AAMC's explanations.

Thanks...

Originally posted by OnMyWayThere
On #23, it states: In the equilibrium constant expression for 2(NH4)2HPO4(s) + H2O(l) ---> 4NH4+(aq) + HPO4-(aq) + H2PO4-(aq) + OH-(aq),..... [H2O] is omitted because the salt is:

A. only weakly basic, and [H2O] is nearly constant

C. only weakly acidic, and [H2O] is nearly constant

So, how does one choose whether the water was acidic or basic in the reactants?

Thanks... I might post some more questions if I don't understand AAMC's explanations.

Thanks...

well, it is obviously basic due to the "OH-" on the reactant side of the equation.

Originally posted by Cerberus
well, it is obviously basic due to the "OH-" on the reactant side of the equation.

I can't believe I read this about 5 times and the AAMC solutions and just realized it asks what the salt is, not the water. I thought it asks if the water was acting as a acid or base.

Thanks...

It asks what the salt is, the salt (NH4)2HPO4(s) dissociates to form 4NH4+(aq) + HPO4-(aq) + H2PO4-(aq) + OH-(aq). Since H2PO4 and OH- are formed (it accepts a proton from H20) it is a base by the def. of a base (a proton acceptor, producer of OH-). The product had it been acitic would have involved PO4- and H3O+.

1 user
Originally posted by OnMyWayThere
I can't believe I read this about 5 times and the AAMC solutions and just realized it asks what the salt is, not the water. I thought it asks if the water was acting as a acid or base.

Thanks...

heh, hate it when that happens