Question

1. The Ka for HF is 6.9 x 10-4. What is the value of Kb for F-? (Show your calculations)

2. The Kb for NH3 is 1.8 x 10-5. What is the value of Ka for NH4+? (Show your calculations)

3. From what acid and what base were the following salts made and what would you expect the pH of the salt solution to be (> 7, =7 or <7)? Why?

Rb3PO4

NH4I  

Answer 1

Ka x Kb = Kw

Question 1: Kb = Kw/Ka

Kb = 1x10^-14/6.9x10^-4=1.44*10^-11

Question 2: Ka = Kw/Kb

Ka = 1x10^-14/1.8x10^-5=5.5*10^-10

3)

a)The compound rubidium phosphate is not an acid. In fact a solution of this compound will be a little bit basic, not acidic.

It is an example of a salt hydrolyzing (reacting with water).
Rubidium, being an alkali metal, does not hydrolyze.
Phosphate ion reacts with water to make HPO4^2- and hydroxide ion:

PO4^3- + HOH <==> HPO4^2- + OH-

b)when NH4I is dissolved in water,it undergoes partial hydrolysis

NH4I + H2O <==> NH4OH + H(+) + I(-)

NH4OH being a very weak base,remains almost undissociated in aq. soln, however HI being a strong acid, remains almost wholly ionised as H(+) & I(-) ions.

Thus the concentration of H(+) ions in soln remains greater than concentration of OH(-) ions,& the soln becomes acidic.

ie, in the soln, [H(+)] > [OH(-)]
so, the soln is acidic