17:04 session.masteringchemistry.com/m.... Item 9 Masteri 9 of 21 You wish to pr

Question

17:04 session.masteringchemistry.com/m.... Item 9 Masteri 9 of 21 You wish to prepare a buffer solution with pH 9.45. Part A How many grams of (NH4) SO4 would you add to 430 mL of 0.257 M NH3 to do this? Assume that the solution's volume remains constant Express your answer to two significant figures and include the appropriate units. Submit Request Answer Incorrect; Try Again; 4 attempts remaining Part B Part complete Which buffer component would you add to 0.100 L of the buffer in part A to change its pH to 9.30?

Explanation / Answer

a)

pH of basic buffer = pka -log((NH4^+/NH3)

    pka of NH4+ = 9.3

    No of mol of NH3 taken = 0.43*0.257 = 0.11 mol

    No of mol of NH4+ = ?

    9.45 = 9.3 - log(x/0.11)

   x = 0.0779

amount of (NH4)2SO4 = (0.0779/2)*132.2 = 5.15 g

c) to decreases, the pH (NH4)2SO4 must be added.

pH of basic buffer = pka -log((NH4^+/NH3)

    pka of NH4+ = 9.3

    No of mol of NH3 taken = 0.09*0.257 = 0.02313 mol

    No of mol of NH4+ = ?

    9.45 = 9.3 - log(x/0.02313)

   x = 0.01637

after addition of excess (NH4)2SO4

9.3 = 9.3 - log(x/0.02313)

   x = 0.02313

No of mol of excess (NH4)2SO4 must be taken = (0.02313-0.01637)/2 = 0.00338 mol

mass of (NH4)2SO4 must be taken = 0.00338*132.2 = 0.45 g

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