Question

Some compounds that are used as fertilizers are NH3, NH4NO3, and (NH4)3PO4. What effect would each of these compounds have on the pH of ground water (that is, water that is in the soil and in which the compound would dissolve)? For compounds with one ion that hydrolyzes to form H3O+ and another ion that hydrolyzes to form OH-, you will need to determine which ion hydrolyzes to a greater extent by comparing the Ka value of the H3O+-generating ion to the Kb of the OH--generating ion (you will need to look up the Ka of its conjugate acid and convert it to a Kb).

Answer 1

NH3 is base , it increases OH- concentration , hence pH increases . The reactionis shown below

NH3 (aq) + H2O (l) <---> NH4+(aq) + OH- (aq) ,

NH4NO3 is salt having NH4+ and NO3- .

it is formed by strong acid HNO3 and weak base NH3.

Salt formed by strong acid and weak base is acidic and it increases H3O+ conc more and thus pH decreases

HNO3 + NH3 ---> NH4NO3

Ka of HNO3 is 0.04 , hence Kb of NO3- = 10^-14/0.04= 2.5 x 10^-13

Kb of NH3 = 1.8x10^-5 , hence Ka of NH4+ = 10^-14 / 1.8x10^-5 = 5.55 x 10^-11

hence NH4+ is stronger acid compared to NO3- base property, hence solution is acidic .

(NH4)3PO4 has NH4+ and PO4 3- ,

Kb of PO43- = 0.0208 , Ka = of NH4+ is 5.55 x 10^-11 ,

hence solution is basic since Kb of PO43- is higher than Ka of NH4+