I really don't understand this one question on my homework (deals with pH/acids/bases)?

Question

The pH of 100 mL of a 0.10M aqueous solution of Compund Z is 11.30.

1. Is Z an acid, a base, neither, or cannot be determined?

2. Write the relevant Brownsted-Lowry equation showing how Z behaves in aqueous solution, and calculate the value for the equilibrium constant for the reaction shown by the equation.


??? I really don't get how I'm supposed to approach this...

Thanks

Answer 1

1. Z is a base, because the pH is 11.30, which is alkaline (7.00 is neutral, below 7.00 is acidic).

2. Z+ H2O ===> HZ+ + OH-
In the Bronsted theory, acids are proton donors, and bases are proton acceptors. Z is a Bronsted base, which accepts a proton from water to become the conjugate acid HZ+ (assuming Z was neutral to begin with LOL). OH- is left, called the conjugate base, which gives a "high" pH.

Answer 2

1. Compound Z is definitely a base. Aqueous substances w/ pH above 7 are basic.

2. The equilibrium of cpd Z in water is as follows:
Z + H2O <=> Z:H+ + OH-
Z:H+ is the conjugate acid of base Z and OH- is the conjugate base of acid H2O.
The equilibrium constant is expressed as:
Keq = [Z:H] [OH] / [Z]
As an example, the base ammonia (NH3), behaves exactly the same as the compound in your problem.