Question

The salt ammonium nitrate, NH4NO3(s), completely dissociates into NH4+(aq) and NO3−(aq) in an aqueous environment. The NO3−ion has no acid or base properties, but the ammonium ion is the conjugate acid of the weak base, ammonia.

Part a. Write a balanced chemical equation showing how the ammonium ion reacts with water to affect the pH of the resulting solution (you can ignore the nitrate ion in your balanced equation).

Part b. Find the pH of a 0.100 M solution of ammonium nitrate, NH4NO3. The Kbfor ammonia is 1.8×10−5.      Ans. pH = 5.13

Answer 1

_{PART A}  

_{It is weakly acidic in nature because it release hydronium ion.}

_{Part b---}

K_a for given equation is

K_a = where water is in exess so concentration water is one

Now consider   

and     

therefore    given concentration of ammonium ion is 0.100M

put these value in the formula of K_a and we get

K_a= x²/0.100-x

as ammonium nitrate is weak acid so the value of x is very small as compare to 0.1 so x can be neglected

therefore K_a= x²/0.1 ---(1)

Now given that K_b= 1.8 × 10^-5

  K_a= K_w/K_b

  =1×10^-14/1.8×10^-5

=5.5555×10^-10

Now put this in equation (1)

5.5555×10^-10 = x²/0.1

x²= 5.5555×10^-11

x = 0.0000074535

we have considered x =

Now according to definition of pH.

pH = -log

therefore pH = -log[x]

pH = -log(0.0000074535)

pH = 5.127 ~ 5.13