Question

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

System is m(NH3)/m(NH4Cl)

Buffer has a pH=9.19.

Answer 1

Henderson's equation for Basic Buffer is

pH = pKa + log [ B ] / [ BH⁺]

In our case, pH = pKa + log [ NH₃] / [ NH₄Cl]

9.19 = 9.245 + log [ NH₃] / [ NH₄Cl]

log [ NH₃] / [ NH₄Cl] = 9.19-9.245 = - 0.055

[ NH₃] / [ NH₄Cl] = 10 ^{- 0.055} = 0.881

i e   [ NH₃] = 0.881 x [ NH₄Cl]

We can write , No of moles of NH₃ = 0.881 x No of moles of NH₄Cl

Molar mass of NH₃ = 14.01 + ( 3 x 1.0079) = 17.03 g / mol

Molar mass of NH₄Cl = 14.01+ ( 4 x 1.0079) + 35.45 = 53.49 g / mol

Assume No of moles of NH₃ = 1 mol then No of moles of NH₄Cl = 1.14 mol.

Mass of 1 mol NH₃ = 1 mol x ( 17.03 g / 1 mol ) = 17.03 g

Mass of 1.14 mol NH₄Cl = 1.14 mol x ( 53.49 g / 1 mol) = 60.98 g

Therefore, Ratio of ( mass of  NH₃ / mass of NH₄Cl ) = 17.03 g / 60.98 g =0.2793