1. A buffer solution contains 0.408 M ammonium bromide and 0.313 M ammonia . If

Question

1. A buffer solution contains 0.408 M ammonium bromide and 0.313 M ammonia.

If 0.0353 moles of sodium hydroxide are added to 225 mL of this buffer, what is the pH of the resulting solution ?
(Assume that the volume change does not change upon adding sodium hydroxide)

pH =

2. A buffer solution contains 0.222 M ammonium bromide and 0.369 M ammonia.

If 0.0314 moles of hydrochloric acid are added to 150 mL of this buffer, what is the pH of the resulting solution ?
(Assume that the volume change does not change upon adding hydrochloric acid)

pH =

Explanation / Answer

1)

moles of ammonium bromide = 0.225 L x 0.408 M =0.0918

Moles of ammonia = 0.225 L x 0.313 M = 0.070 moles

NaOH reacts with ammonium bromide to form ammonia and NaBr

Moles of NH3 formed = 0.0353 moles

Total moles of NH3 = 0.1057 moles

pKb of NH3 = 4.75

pH = 9.25 + log (0.1057/0.0918) = 9.31

2)

moles of ammonium bromide = 0.150 L x 0.222 M =0.0333 moles

Moles of ammonia = 0.150 L x 0.369 M = 0.05535 moles

HCl reacts with ammonia to form ammoniam chloride

Moles of ammoniam chloride = 0.0314 moles

Total moles of NH4+ = 0.0647 moles

pKb of NH3 = 4.75

pH = 9.25 + log (0.05535/0.0647) = 9.18

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