Question

Which of the following buffer systems would be the best choice to create a buffer with pH = 9.00?

HF/KF, HNO2/KNO2, NH3/NH4Cl, HClO/KClO

Answer this:

For the best system, calculate the ratio of the masses of the buffer components required to make the buffer.

m(NH3)/m(NH4Cl) = ?

Express your answer using two significant figures.

Answer 1

A buffer is the mixture of acid or base and their salt (or conjugate base), in a proportion such that pH change is in the range -1>pH>+1

At a given pH,the best buffer is the one that has pka value in the narrow range, of pH1.

pH=pkalog [base]/[acid] [henderson-hasselbach equation]

for [base]=[acid]

pH=pka(most effective buffer)

for [base]/[acid] =0.1 to 10

pH=pka 1

For the given buffer syatems,

1)HF/F- ,pka=3.14

2)HNO2/KNO2,pka=3.39

3)NH3/NH4Cl,pka=9.25

4)HClO/KClO,pka=7.53

NH3/NH4Cl,pka=9.25 has pka within pH=9.0 +/-1 range ,so its the best buffer at pH=9.0

pH=pkalog [NH4+]/[NH3] [Since NH3 is the base and NH4+ is the conjugate acid]

or,9.0=9.25log [NH3]/[NH4+]

or,-0.25=log [NH3]/[NH4+]

[NH3]/[NH4+]=10^-0.25=0.562:1

taking molar ratio to be 0.562 mol :1mol

molar mass of NH3=17.031 g/mol

molar mass of NH4+=18.039 g/mol

ratio of mass=m[NH3]/m[NH4+]=0.562mol*(17.031 g/mol)/1mol*(18.039)=9.571/18.039=0.53

m[NH3]/m[NH4+]=0.53

if m stands for mass,then 0.53