31. A 25.00 mL pipetful of hard water is placed into a 100.00 mL volumetric flask and diluted to the mark with distilled (soft) water. 50.00 mL of this resulting solution requires 32.00 mL of a 0.0250 M EDTA solution to reach the equivalence point. Calculate the hardness of the original water sample in ppm CaCO₃ (MW: = 100.00)
ANS: 6400 ppm

32. The lead in a 0.3000 gm sample of impure lead ore is precipitated as yellow PbCrO₄. This precipitate is filtered, washed and then dissolved in 1 M HNO₃, giving a solution of Pb²⁺ and dichromate ions, Cr₂O₇^2-. The Cr₂O₇^2- is now titrated with a 0.1000 M Fe²⁺  SO4^2- titrant, 17.28 mL of this titrant needed to reach the equivalence point. In the titration Cr₂O₇^2- is converted to Cr³⁺ while Fe²⁺ is converted to Fe³⁺. Calculate the % Pb in the sample. (MW: PbCrO₄ = 323.2; Pb = 207.0) ANS: 39.74%
 

33. A 10.00 mL sample of vinegar, an aqueous solution of acetic acid (HC₂H₃O₂), is titrated with 0.5062 M NaOH, and 16.58 mL is required to reach the equivalence point.
a) What is the molarity of the acetic acid? ANS: 0.8393 M
b) If the density of the original vinegar solution is 1.006 g/cm3, what is the mass percent of acetic acid in the vinegar? ANS: 5.011%
 

34. Acetylsalicylic acid is the active ingredient in aspirin. It took 35.17 mL of 0.5065 M sodium hydroxide to react completely with 3.210 g of acetylsalicylic acid. Acetylsalicylic acid has one acidic hydrogen. What is the molecular weight of acetylsalicylic acid? ANS: 180.2 g/mol

35. A 25.0-mL sample of 0.100 M lactic acid (HC₃H₅O₃, pKa = 3.86) is titrated with 0.100 M NaOH solution. Calculate the pH after the addition of 0.0 mL, 4.0 mL, 8.0 mL, 12.5 mL, 20.0 mL, 24.0 mL, 24.5 mL, 24.9 mL, 25.0 mL, 25.1 mL, 26.0 mL, 28.0 mL, and 30.0 mL of the NaOH. Plot the results of your calculations as pH versus milliliters of NaOH added.
ANS: at e.p. pH = 8.28

Clueless:

This is the calculation associated with a complexometric titration such as the one you've done in the lab for your water-hardness determination.

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Hints:

The two points of interest here are 1) the use of a polydentate ligand, EDTA, which makes the conversion between titrant and analyte a trivial step, and 2) the preliminary dilution step, which requires attention when backtracking through the titration data to get to the hardness value for the original solution.

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Clueless:

This is a redox determination, albeit a complicated one. The idea is to determine the %Pb by ultimately performing a titration of I2 with a standardized solution of S2O32-. (You performed this very titration when you carried out your determination of %Cu in a sample)

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Hints:

The idea here is that the Pb goes through numerous partners in the preliminary chemistry until you generate an amount of I2 which can ultimately be traced back through a series of balanced equations to the original Pb. It is the preliminary chemistry which is the headache here and which will require some time and thought.

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Clueless:

Part a) A pretty standard titration calculation, leading to the concentration of acetic acid.

Part b) This calculation is for the mass% of acetic acid (weight/weight basis)

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Hints:

Part b) you need to employ the density so that you can change a weight/vol unit (molarity) into a weight/weight unit (mass %)

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Clueless:

This one is a straightforward titration calculation, but with the one wrinkle that the Molecular Weight of the analyte (aspirin) is unknown.

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Hints:

Set up the template which calculates the % aspirin. The points to note here are:
1) you assume the aspirin is pure, so the template will be set up to yield % aspirin = 100
2) make the Molecular Weight of aspirin the unknown value X. When you set up the template, if you set % aspirin = 100 and fit in the other data, you'll have only 1 unknown to solve for.

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Clueless:

This is just another example of drawing a titration curve. Refer to your notes.

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Hints:

Recall that this is the titration curve for a Weak Acid, so you're best directed to use the pH line at all stages of titration except for the very beginning and the equivalence point. Beyond the equivalence point, of course, you're only adding strong base again.

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