Question

A. Which of the following buffer systems would be the best choice to create a buffer with pH = 9.05?
HF/KF
HNO2/KNO2
NH3/NH4Cl (correct answer)
HClO/KClO

B. For the best system, calculate the ratio of the masses of the buffer components required to make the buffer. Express your answer using two significant figures.

m(NH3)/m(NH4Cl) =

Answer 1

A. Which of the following buffer systems would be the best choice to create a buffer with pH = 9.05?
We need a pKa value of the acid near pH = 9.05; pKa = 9 approx

HF/KF -- > pKa near 3.4, ignore
HNO2/KNO2 --> pKa 3.39, ignore
NH3/NH4Cl --> pKa = 9.25; this is approx pH = 9.05, possible answer
HClO/KClO --> pKA = 7.2 approx

therefore; choose NH3/NH4Cl, since NH4+ acid will hae pKa = 9.25 approx, therefore the best option to make a buffer

calculate mass ratio:

m(NH3)/m(NH4Cl) =

pH = pKa + log(NH3/NH4Cl)

9.05 = 9.25 + log(NH3/NH4Cl)

(NH3/NH4Cl) = 10^(9.05-9.25)

(NH3/NH4Cl) = 0.63095

change to mass

1 mol of NH3 = 17 g

1 mol of NH4Cl = 53.491 g

assume a basis of 1 mol of NH4Cl

(NH3/NH4Cl) = 0.63095

NH3 = 0.63095*NH4Cl

1 mol of NH4Cl --> 0.63095 mol of NH3

then

mass of NH4Cl = 53.491 g

mol of NH3 = 0.63095*17 = 10.72615 g

ratio --> NH3/NH4Cl = 10.72615 /53.491 = 0.200 g of NH3 per g of NH4Cl