Q Acid-Base Indicators:
Spectrophotometric Methods
Dr. Prem D. Sattsangi
Copyright © 2009
1. Molar Absorptivity
• Define Molar Absorptivity
A. Calculate the molar absorptivity with the following data
• 1.25 x 10-4 M solution has 0.45 O.D. absorbance at the λmax (455 nm).
B. If the molar absorptivity of a compound is 33,350, calculate the
absorbance of a 6.5 x 10-5 M solution.
2. Moles
• Calculate moles of NaOAc present in 5.00 mL of a 0.154 M soln.
• n=MxV
3.
• Calculate the [H3O+] for a buffer solution containing 0.00555 M AcO(= NaOAc) and 0.00333 M AcOH. (Given Ka for AcOH = 1.75 x 10-5)
4.
• Given the following data, calculate the Ka for a weak acid.
• Ka = [H3O+] [B-]
[HB]
• [H3O+] = 3.45 x 10-5M,
[B-] = 0.0045M,
[HB] = 0.01 M
5.
• For a buffer, pH = pKa + log [X-]
[HX]
• At what concentration of [X-] and [HX] would pH = pKa?
6.
• During a titration experiment we take 25.00 mL of AcOH and add
25 mL of water to make it 50 mL. If the first pH reading is 2.45,
(Given: pH = -log [H3O+])
a) What is the [H3O+] of the diluted solution?
b) What would be the [H3O+] of the solution before dilution? Equation
for dilution must be used V1 x M1 = V2 x M2
c) Calculate the Ka of acetic acid using this data.
AcOH + H2O ↔ H3O+ + AcO- (Given: [AcOH] = 0.115 M)
Ka = [H3O+] [AcO-]
[AcOH]
7.
• If the molarity of NaOH is 0.113 M, and it takes 7.5 mL of phosphoric
acid, calculate the molarity of phosphoric acid using the following
data on its titration.
• H3PO4 + NaOH  NaH2PO4 + H-OH …Eq. 6
• NaOH
• H3PO4
(0.113 M), 7.5 mL
(? M), 25.00 mL
a) At what volume would you expect the 3rd equivalence point?
b) Molarity of H3PO4: [M = n/V*L]
8.
• H3PO4 can ionize in 3 steps. Write chemical equations for ionization
steps 1 and 2.
• Write expression for
• Ka2 =
9.
• Write an expression for Ksp of Ag2SO4.
10.
• KHT(s) is saturated in distilled water. The saturated solution (25.00
mL) required 16.5 mL of 0.0402 M NaOH to reach the end point.
a) Write an equation for this titration.
b) Calculate the solubility of KHT in distilled water. _______M
c) Calculate Ksp for KHT [=________ (Unit _____)]
11.
• An U.K. solution was given in 100 mL volumetric. After making it up
to the mark, it was saturated with KHT, filtered and titrated. If 25.00
mL of the filtrate took 4.50 mL of 0.0402 M NaOH, calculate the
molarity of the unknown. [Ksp for KHT = 5.1 x 10-4 M2]
12.
• In a titration of solubility of KHT in 0.125 M KNO3, one used 4.53 mL
of 0.0402 M NaOH solution for 25.00 mL of the saturated filtrate.
a) Calculate the solubility of KHT in 0.125 M KNO3. (=_____M)
b) Ksp for KHT in presence of 0.125 M KNO3.
Molar Absorbtivity ”ε”
ε
=
Absorbance
Concentration(M)
Calculate the molar absorptivity “ε” for
morphine, if a 3.68 x 10-3 M solution
has a 0.45 O.D. absorbance at the
‫גּ‬max (328nm)
Calculating Moles of a Solution
• n=MxV
• n = number of moles of solution
• M = molarity of the solution
• V = volume of the solution in liters (L)
• Calculate moles of NaOAc (sodium
acatate) present in 5.00mL of a 0.250M
solution.
Calculation of Hydrogen Ion
Concentration
• [H3O+] =
Ka x
[AcOH]
[AcO-]
• Calculate the [H3O+] for a buffer solution
containing 4.25 x 10-3 M AcO- (acetate ion)
and 2.25 x 10-3 AcOH (acetic acid).
[Ka for AcOH = 1.75 x 10-5]
Bromocresol Green Indicator