Question

Calculate the pH of a buffer solution prepared by mixing 1.35mL of concentrated ammonia (14.8M) with 1.07 g of solid ammonium chloride in a 1.00-L volumetric flask. The acid ionization constant of the ammonium ion is 5.6×10-10.

Answer 1

Let's calculate molar concentration of ammonia in 1 L solution.

We have diluted 1.35 ml of 14.8 M ammonia solution to 1000 ml . Hence, we can use dilution formula to calculate molar concentration of ammonia in 1 L solution.

We have dilution formula, C _stock V _stock = C _dilute V _dilute

C _dilute = C _stock V _stock /  V _dilute

C _dilute = 14.8 M 1.35 ml / 1000 ml = 0.01998 M

[ NH ₃ ] = 0.01998 M

Now, calculate molar concentration of ammonium chloride.

[ NH₄Cl ] = No. of moles of NH₄Cl / volume of solution in L

No. of moles of NH₄Cl = Mass of  NH₄Cl / Molar mass of  NH₄Cl

Molar mass of NH₄Cl = 14.0067 + ( 4 1.0079 ) + 35.45 = 53.49 g / mol

No. of moles of NH₄Cl = 1.07 g / ( 53.49 g /mol ) = 0.0200 mol

[ NH₄Cl ] = 0.0200 mol / 1 L = 0.0200 M = [ NH₄⁺ ]

pH of basic buffer solution is calculated by using Henderson's equation.

pH = pKa of conjugate acid + log [ Base ] / [ Conjugate acid ]

pH = pKa of NH₄⁺ + log [ NH ₃ ]  / [ NH₄⁺ ]

pH = - log Ka + log [ NH ₃ ] / [ NH₄⁺ ]

pH = -log ( 5.6 10 ^-10 ) + log 0.01998 / 0.0200

pH = 9.25 + log 0.01998 / 0.0200

pH = 9.25 - 0.0004345

pH = 9.25043

i e pH of buffer solution = 9.25