Item 3 3 of 7 > Part A Constants| Periodic Table What mass of HCl can this buffe

Question

Item 3 3 of 7 > Part A Constants| Periodic Table What mass of HCl can this buffer neutralize before the pH falls below 9.00? A 110.0 mL buffer solution is 0.100 M in NH3 and 0.135 M in NH4Br Express the mass in grams to three significant figures You may want to reference (Pages 740-751) Section 17.2 while completing this problem m= Submit Part B If the same volume of the buffer were 0.260 M in NH and 0.390 M in NH4Br, what mass of HCI could be handled before the pH falls below 9.00? Express the mass in grams to three significant figures

Explanation / Answer

part A)

moles of NH3 = 110 x 0.1 / 1000 = 0.011

moles of NH4+ = 110 x 0.135 / 1000 = 0.01485

pKa = 9.26

pH = pKa + log [NH3 - C / NH4+ + C ]

9.0 = 9.26 + log (0.011 - C / 0.01485 + C)

0.011 - C / 0.01485 + C = 0.550

0.011 - C = 0.550 C + 8.16 x 10^-3

2.84 x 10^-3 = 1.550 C
C = 1.83 x 10^-3

mass of HCl = 1.83 x 10^-3 x 36.5 = 0.0669 g

mass of HCl = 0.0669 g

part B)

part A)

moles of NH3 = 110 x 0.260 / 1000 = 0.0286

moles of NH4+ = 110 x 0.390 / 1000 = 0.0429

pH = pKa + log [NH3 - C / NH4+ + C ]

9.0 = 9.26 + log (0.0286 - C / 0.0429 + C)

0.550   = (0.0286 - C / 0.0429 + C)

0.0286 - C   = 0.550 C + 0.0236

5.005 x 10^-3 = 1.550 C
C = 3.229 x 10^-3

mass of HCl = 0.118 g

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