Question

Calculate the volume of concentrated ammonia and the weight of ammonium chloride you would have to take to prepare 100 ml of a buffer at ph 10.00 if the final concentration of ammonium chloride is to be 0.200 M( The molarity of concentrated ammonia is 14.8)

Answer 1

Ka of ammonium = 5.8 x 10^-10

pKa = -log (5.8 x 10^-10) = 9.24

[NH4+] = 0.200 M

pH of buffer = 10.00

According to Henderson Hasselbalch equation:

pH = pKa + log([NH3]/[NH4+])

10.00 = 9.24 + log([NH3]/[NH4+])

0.76 =  log([NH3]/[NH4+])

[NH3]/[NH4+] = 10^0.76

([NH3]/[0.200]) = 5.75

[NH3] = 5.75*0.200

[NH3] = 1.150 M

Volume of buffer = 0.1 L

Volume of concentrated NH3 = 0.1 L*1.150 M/14.8 M

= 0.00777 L

= 7.77 mL (Answer)

Moles of NH4Cl to be added = 0.100 L * [NH4+]

= 0.100 L * 0.200 M

= 0.02 mol

Mass of NH4Cl to be added = Moles * Molar mass

= 0.02 mol*53.49 g/mol

= 1.07 g (Answer)