A buffer is prepared by partially titrating 500.0 mL of 0.250 M ammonia, NH 3 (K

Question

A buffer is prepared by partially titrating 500.0 mL of 0.250 M ammonia, NH3 (Kb = 1.75 x 10-5)  with 1.25 M HCl until a pH of 9.500 is reached.  

(a) Calculate the ratio of NH3 to NH4+ required to achieve the target pH.

(b) How many moles of NH3 are initially present?  

A buffer is prepared by partially titrating 500.0 mL of 0.250 M ammonia, NH3 (Kb = 1.75 x 10-5) with 1.25 M HCl until a pH of 9.500 is reached. with 1.25 M HCl until a pH of 9.500 is reached. Calculate the ratio of NH3 to NH4+ required to achieve the target pH. How many moles of NH3 are initially present?

Explanation / Answer

pKb = 4.757

pH = 9.5

=> pOH = 4.5

pOH = 4.5 = pKb + log (Salt / Base)

=> 4.5 = 4.757 + log (Salt / Base)

=> Salt / Base = 0.5534

=> NH3 / NH4+ = 1 / 0.5534 = 1.807

b) Moles of NH3 = 0.5 x 0.25 = 0.125 moles

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