[tashkent]

Hello everyone!

I'm preparing for my GRE chemistry exam and I need help in this particular GRE problem:

3) In  which of the ff reactions is the equilibrium farthest to the left?

a) P4O10 + xH2O = 4H3PO4(aq)
b) AlCl3 + xH2O = Al[(H2O)6]+ 3 Cl-(aq)
c) Li2O + xH2O = 2Li+(aq) + 2OH-(aq)
d) H2S + H2O = H3O+ + HS-(aq)
e) CaC2 + H2O = Ca+2 (aq) + 2OH-(aq) + C2H2(g)

The answer is D, but I have no idea why.  I will appreciate any explanation for this.  Thanks!

Regards,
Tashkent

[Demotivator]

Only through experience.
Need to recognize H2S is a weak acid.
The others are strong bases or stronger acid anhydrides.

the last one (e) is easy to discount because C2H2 gas is formed, so as gas is evolved, the equilibrium keeps shifting to the right.
(c) can be discounted because oxides of groupI are strong bases.
a) Since Phosphoric is a weak acid, it prefers being protonated and therefore it's goes to the right.
b) AlCl3 is a lewis acid. Al3+ is easily hydrated.

[tashkent]

Demotivator,

Thank you so much for the analysis!  I will try to study again the reasons that you gave.  I will write again if I have further questions...

Thanks again,
Tashkent