Question

Which solution should be mixed with 50.0 mL of 0.100M ammonia, (NH₃), to make

an effective buffer?

Select one:

a. 50.0 mL of 0.200 M HCl

b. 25.0 mL of 0.200 M HCl

c. 50.0 mL of 0.100 M HCl

d. 25.0 mL of 0.100 M HCl

Answer 1

A buffer is a mixture of weak base and the salt of the weak base (or weak acid and its salt)
The buffers work most effectively when the concentration of the salt and acid are in the ratio 1:1
It will take an equal amount of acid or base to change the pH of the buffer the same amount.
change in P^H = log ([salt] / [acid])
change in P^H is minimum when [salt] = [acid]

NH₃ (weak base)+ HCl(strong acid) -------------------------> NH₄Cl (salt of weak base)
Number of moles of NH₃ =0.05lit * 0.100moles/lit =5*10^-3moles
So, in order to make effective buffer you should mix 5*10^-3 moles of HCl
a) 0.05 lit * 0.2moles/lit =0.01 moles
b)0.025lit * 0.2moles/lit = 5*10^-3 moles
c) 0.05lit * 0.1moles/lit = 5*10^-3moles
d) 0.025lit * 0.1moles/lit = 2.5*10^-3moles

We cannot choose option C , because if volume of HCl is > or = volume of NH₃
you may run out of acid which will not help to give effective buffer.
So, the right answer is Option B