Question

B. Preparation of Ammonia-Ammonium Buffer Solution Note that an ammonia buffer contains the conjugate base, ammonia, and its conjugate acid, the ammonium ion, NH typically added as NH,CI. Accordingly, the pk, value stated in Eq. (7) is that of the buffer's conjugate acid, NH.. For ammonia, pk, -log (1.8 x 107 4.74. (Note that the fact that the pk for NH, is the same value as the pk, for acetic acid is pure coinci dence.) Thus the pk, value for ammonia buffer solutions is pk = 14 -4.74 - 9.26. Consider the conditions that will yield 100 mL of an ammonia buffer solution with a pH<9.26. Equation (7) indicates that, if the log term is to be zero, the concentrations of the conjugate base (ammo- nia) and the conjugate acid (ammonium ion) must be equal. Because the conjugate base, delivered as ammonium chloride, will be dissolved in the same volume that contains 10 mmol of ammonia, we must add 10 mmol of NH Cl to the acid. The resulting buffer volume may be slightly larger than the original 100 mL of acid because of the volume taken up by the ammo nium chloride in solution. NH,Cl is available most often as the anhydrous salt, but students should verify the formula and molecular weight from the reagent bottle. The 10 mmol mass of ammonium chloride must be determined using the correct molecular weight. 6. Measure 100 mL of 0.10 M ammonia in a graduated cylinder. 7. Transfer it to a 250-ml beaker and measure and record the pH. The ammonium chloride will be added to this solution. Save it and do not contaminate it. 8. Calculate the quantity of ammonium chloride needed to make the ammonia buffer. Record your calculations in lab notebook. Make sure to obtain the correct information as to the sodium acetate from the label on the bottle. 9. To 100 mL of 0.10 M ammonia, add 10 mmol of ammonium chloride and stir the mixture until all of the salt dissolves. 10. Measure and record the pH. Effect of Addition of Strona

With this how I calculate the Ammonia solutions pH and Ammonia Buffer solutions ph

Answer 1

Ammonia solutions pH:

NH3 is weak base.

PKb of NH3 = 4.745

Using

pOH = 1/2 * [pKb - log C]

or

POH = 1/2 * [4.745 - log (0.10)]

or

pOH = 2.87

thus

pH = 14 - pOH = 14 - 2.87 = 11.13

thus

pH of ammonia solution = 11.13

and

Ammonia Buffer solutions ph:

100 ml of 0.10 M NH3 = 100 * 0.10 = 10.0 mmole.

and

mmole of NH4Cl = 10 mmole

Using Henderson equation,

pOH = pKb + log [salt] / [base]

or

pOH = 4.745 + log (10 / 10.0)

or

pOH = 4.745

thus

pH = 14 - 4.745 = 9.26

thus

pH of buffer solution is 9.26