25.00 mL sample of 0.08000 M NH3 (Kb = 1.8 × 10-5) is being titrated with 0.1000 M HCl. a. Calculate the initial pH of the base. b. Calculate the pH at 5.00 mL before the equivalence point. c. Calculate the pH at the equivalence point. d. Calculate the pH at 5.00 mL after the equivalence point.

Chemistry: An Atoms First Approach
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Chapter14: Acid- Base Equilibria
Section: Chapter Questions
Problem 106CWP: Consider the titration of 100.0 mL of 0.200 M HONH2 by 0.100 M HCI. (Kb for HONH2 = 1.1 108.) a....
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25.00 mL sample of 0.08000 M NH3 (Kb = 1.8 × 10-5) is being titrated with 0.1000 M HCl.

a. Calculate the initial pH of the base.

b. Calculate the pH at 5.00 mL before the equivalence point.

c. Calculate the pH at the equivalence point.

d. Calculate the pH at 5.00 mL after the equivalence point.

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Step 1

(a). molarity of NaOH [OH-]=0.080000M pOH=log[OH-]therefore,  pOH=-log 0.080000M pOH=1.097pH=14-1.097pH=12.903 (b).  At equivalent point moles of number in acid (HCl)= moles of number base (NaOH)so, at equivalent point volume of HCl=molarity of NaOH ×volume of NaOHmolarity of HCl 0.080000M×25 mL0.1000M=20mL  before At equivalence point  volume of HCl=20mL-5mL=15mL number of moles of NaOH= molarity(NaOH)× volume (L)=0.080000M×0.025 mL=0.002 molsimilarly, for HCl,  number of moles of HCl =molarity(HCl)× volume (L)=0.1000M×0.015L =0.0015 mol  total volume =25mL+15mL =40mL =0.040mL [OH-]=moles of NaOH- moles of HCltotal volume in L=0.002 mol -0.0015 mol0.040 =0.0125M therefore the pOH= -log 0.0125MpOH=1.90 pH=14-1.9012.10

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