What is pH of buffer.?

Question

A buffer is prepared by adding 5.0g of ammonia (NH3) and 20.0g of ammonium chloride (NH4Cl) to enough water to form 2.50L of solution.

a) what is the pH of this buffer?

b) write the complete ionic equation for the reaction that occurs when a few drops of nitric acid are added to buffer.

c) write complete ionic equation for the reaction that occurs when a few drops of potassium hydroxide solution are added to the buffer.

Answer 1

Shame on your for asking a homework question on yahoo ;-)

I won't give you the answer, but you need to use the Henderson Hasselbach equation. The NH4Cl is the acid and the NH3 is the base.

Answer 2

MM(NH3)= 17 g/mol
5.0 g / 17 =0.294 moles
Molarity =0.294/2.5 =0.118 M

MM(NH4Cl)=53.4 g/mol
20.0/53.4 = 0.37 moles
Molarity =0.37/2.5= 0.150

NH3 + H2O <> NH4+ + OH-
initial concentration
0.118.... ........... .0.150.....0

at equilibrium
0.118-x.... .... ......0.150+x...x

K = 1.81 10^-5 = (0.150 +x)(x) / 0.118-x

neglecting x where it adds or subtracts we get

x = 0.0000142 M Concentration OH-

To get H+ we need to use Kw

(H+)= Kw /(OH-)= 10^-14/ 0.0000142 = 7.04 10^-10

pH= 9.15


b) The reaction is
NH3 + H+ >> NH4+

the add of H+ will decrease NH3 and will increase NH4+

c) The reaction is
NH4+ + OH- >>NH3+H2O
and the add of OH- reduces the moles of NH4+ and increases the moles of NH3