Question

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

an effective buffer?

50.0 mL of 0.200 M HCl

25.0 mL of 0.200 M HCl

50.0 mL of 0.100 M HCl

25.0 mL of 0.100 M HCl

Answer 1

NH3 is base here

So, we need some conjugate acid NH4+

So, if we add HCl such that number of moles of HCl is less than number of mol of NH3, then some of NH3 will convert to NH4+.

So, finally we will have NH3 and NH4+

mol of NH3 present = 0.100 M * 50.0 mL = 5 mmol

option 1:

mol of HCl = 0.200 M * 50.0 mL = 10 mmol

option 2:

mol of HCl = 0.200 M * 25.0 mL = 5 mmol

option 3:

mol of HCl = 0.100 M * 50.0 mL = 5 mmol

option 4:

mol of HCl = 0.100 M * 25.0 mL = 2.5 mmol

option 4 is correct

Answer: 25.0 mL of 0.100 M HCl