Joanna is performing a reaction that generates a moderate amount of hydroxide. T

Question

Joanna is performing a reaction that generates a moderate amount of hydroxide. To mimic biological conditions (most bodily fluids in living organisms retain a relatively constant pH around 7), she knows that she needs to add a buffer to the reaction. Which of the following will MOST effectively neutralize the OH ions produced by the reaction?

Select the correct answer below:

A buffer containing HCl and NaCl in equal concentrations

A buffer containing NH3 and NH4Cl in equal concentrations

A buffer containing H3PO4 and H2PO4, with a slight excess of H3PO4

A buffer containing NH3 and NH4Cl, with a slight excess of NH3

Explanation / Answer

Solution:-

The correct answer is the last one A buffer containing NH3 and NH4Cl, with a slight excess of NH3

because pKa of NH4+ is 9.74 when the ratio of NH4+Cl and NH3 is the same the pH will be 9.74

As pH = pKa + log10[ NH4+/ NH3 ] and ammonia is in the denominator so increase in ammonia will again decrease the ph and it will become close to 7 and when ammonium salt will react with hydroxide it will generate water and ammonia so pH will maintain at 7 but in the first case

The buffer cannot be made by strong acid and a strong base so HCl and NaCl will not work

pKa of phosphoric acid is around 1 or 2 so it will be a very acidic buffer and on increasing, phosphoric acid will again decrease the pH so it is also not a good choice for work.

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