Question

What mass of NH₄Cl will be required to prepare 1.0 L of a buffer solution at pH 9.50 containing 68.00 mL of NH₃ (aq) ( ρ = 0.90 g.cm-3 ; 28 % w/w) which pK_B is 4.75?

Answer 1

volume of NH₃ (aq) = 68.00 mL

density of NH₃ ( ) = 0.90 g.cm^-3 = 0.90 10³ g/L

weight percentage of NH₃ = 28 %

weight of 68.00 mL of NH₃(aq) = 68.00 10 ^-3 L 0.90 10³ g/L

= 61.2 g

Since the solution is 28% w/w

amount of NH₃ present in the solution =

=  

= 17.136 g

molar mass of NH3 = 17g

number of moles of NH₃ present =  

=

= 1.008 moles

concentration of NH₃ in buffer =

=

= 1.008 M

given pH = 9.50

pKb = 4.75

pOH = 14 - pH

= 14 - 9.50

= 4.50

Adding ammonium chloride, NH₄Cl, to an ammonia solution will effectively create a buffer solution that contains ammonia, a weak base, and the ammonium ion, NH⁺⁴, its conjugate acid.

This means that you can use the Henderson - Hasselbalch equation to find the concentration of conjugate base needed

According to Henderson - Hasselbalch equation

concentration of NH₄⁺ ions = 0.566 M

Ammonium chloride dissociates completely in aqueous solution to give

from the equation one mole of ammonium chloride (NH₄Cl) produces one mole of ammonium ion NH₄⁺

therefore concentration of ammonium chloride (NH₄Cl) in the solution = concentration of ammonium ion (NH₄⁺)

[NH₄Cl] = [NH₄⁺] = 0.566 M

molarity of NH₄Cl =

molecular weight of NH₄Cl = 53.5 g