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Procedure for calculating pH of a titration

Procedure for calculating pH of a titration. Volume of titrant needed is based on amount of analyte. HNO 2 ( aq ) + OH - ( aq ) NO 2 - ( aq ) + H2O ( l ). Initial ( M ). 0.01. Change ( M ). [NO 2 - ] =. = 0.05 M.

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Procedure for calculating pH of a titration

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  1. Procedure for calculating pH of a titration Volume of titrant needed is based on amount of analyte

  2. HNO2(aq) + OH-(aq) NO2-(aq) + H2O (l) Initial (M) 0.01 Change (M) [NO2-] = = 0.05 M NO2-(aq) + H2O (l) OH-(aq) + HNO2(aq) 0.200 Equilibrium (M) x2 [OH-][HNO2] = Kb = 0.05-x [NO2-] Exactly 100 mL of 0.10 M HNO2 are titrated with a 0.10 M NaOH solution. What is the pH at the equivalence point ? start (moles) 0.01 0.01 end (moles) 0.0 0.0 0.01 Final volume = 200 mL 0.05 0.00 0.00 -x +x +x 0.05 - x x x pOH = 5.98 = 2.2 x 10-11 pH = 14 – pOH = 8.02 0.05 – x 0.05 x 1.05 x 10-6 = [OH-]

  3. A sample of 0.1276 g of an unknown monoprotic acid was dissolved in 25.0 ml of water and titrated with 0.0633 M NaOH solution. The volume of the NaOH required to get to the equivalence point was 18.4 ml. What is the molar mass of the acid? Moles Base = (0.0633 moles/L)(0.0184 L) = 0.00116 moles Moles Base = Moles Acid @ Equiv. Point MW Acid = 0.1276 g / 0.00116 moles = 110. g/mole After 10.0 ml NaOH solution was added, the pH was 5.87. What is the Ka of the unknown acid? [A-] pKa = pH - log [HA] 10.0 ml NaOH => (0.0633 moles/L)(0.0100 L) = 0.000633 moles = moles A-

  4. Moles HA = (total moles HA) - (moles A- ) = 0.00116 moles - 0.000633 moles = 0.000527 moles [A-] [A-] pKa = pH - log = moles / V [HA] [HA] moles / V 0.000633 Volumes are the same since they are in the same solution = 5.87 - log 0.000527 = 5.87 - 0.079 pKa = 5.79 Ka = 10-5.79 = 1.62 x 10-6

  5. Acid-Base Indicators HIn (aq) H+(aq) + In-(aq) [HIn] [HIn]  10  10 [In-] [In-] Color of acid (HIn) predominates Color of conjugate base (In-) predominates 16.5

  6. The titration curve of a strong acid with a strong base. 16.5

  7. Which indicator(s) would you use for a titration of of HNO2 versus KOH ? Weak acid titrated with strong base. At equivalence point, will have conjugate base of weak acid. At equivalence point, pH > 7 Use cresol red or phenolphthalein Low pKa acids (stronger weak acids) change color @ low pH High pKa acids (weaker weak acids) change color @ high pH Use small amount of indicator so experiment is not altered.

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