Question

What is the pH of a 100mL solution of 1M ammonia and what is the pH if it also contains 0.1 mol of ammonium chloride?

Answer 1

we know that

moles = molarity x volume (L)

so

moles of ammonia = 1 x 100 x 10-3

so

moles of ammonia = 0.1

also given

moles of NH4Cl = 0.1

now

we know that

NH3 and NH4+ form a basic buffer

and

for buffers

according to hasselbach hendersen equation

pOH = pKb + log [conjugate acid / base ]

so

in this case

we get

pOH = pKb + log [NH4+/Nh3]

pK for Nh3 is 4.75

so

using the obtained values

we get

pOH = 4.75 + log [0.1/0.1]

pOH = 4.75 + log 1

pOH = 4.75

now

we know that

pH = 14 - pOH

so

pH = 14 - 4.75

pH = 9.25

so

the pH of the given solution is 9.25