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28 Nov 2019

A 50.00 mL aliquot of a hard water sample is titrated with 15.00 mL of 0.0100 M EDTA, using Arsenazo indicator. A second 50.00 mL aliquot is made strongly alkaline with sodium hydroxide. A precipitate results. Calcein indicator is added to the solution. The sample then requires 10.00 mL of 0.0120 M EDTA for titration. Calculate for this sample (a) the molarity of the calcium present, (b), the molarity of the magnesium present, and (c) the total hardness of the water (as ppm of CaCO3 and MgCO3).

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A student wishes to analyze a seawater sample to determine the amounts of Na+ , Ca2+ and Mg2+ present. She begins with 50.00 mL sample of seawater that she dilutes to 500.00 mL. She removes a 50.00 mL aliquot of the diluted solution, which she analyzes for total cations present by exchanging the cations for H+ on an ion exchange column and titrating the liberated acid with 0.5000 M NaOH. The amount of NaOH required to reach the endpoint was 22.48 mL.

The student then titrates another 50.00 mL aliquot of her diluted seawater solution with 15.31 mL of 0.0100 M EDTA solution at a pH of 10 to the Erio T endpoint.

The third titration, performed on a 50.00 mL aliquot of the diluted seawater solution involves precipitating Mg2+ as the hydroxide and titrating the Ca2+ remaining in solution with 0.010 M EDTA solution. Only 5.15 mL of EDTA were required to reach the end point.

Summary of titration data:

Original seawater = 50.00 mL; diluted seawater = 500.00 mL

Vol. of titration aliquot = 50.00 mL

First titration: [NaOH] = 0.5000 M; V NaOH = 22.48 mL

Second titration: [EDTA] = 0.0100 M; VEDTA = 15.31 mL

Third titration: [EDTA] = 0.0100 M; VEDTA = 5.15 mL

From the titration data, calculate the following (show work):

A). mmol of Ca2+ present in the third 50.00 mL titration aliquot - ans.__________

B). mmol of Ca2+ present in the original 50.00 mL seawater sample - ans.__________

C). mg of Ca2+ present in the original 50.00 mL seawater sample - ans.__________

D). mmol of Ca2+ and Mg2+ present in the second 50.00 mL titration aliquot - ans.__________

E). mmol of Mg2+ present in the 50.00 mL titration aliquot - ans.__________

F). mmol of Mg2+ present in in the original 50.00 mL seawater sample - ans.__________

G). mg of Mg2+ present in in the original 50.00 mL seawater sample - ans.__________

H). meq of Ca2+ and Mg2+ present in the 50.00 mL titration aliquot - ans.__________

I). total meq of Na+ , Ca2+ and Mg2+ present in the 50.00 mL titration aliquot - ans.__________

J). meq of Na+ in the 50.00 mL titration aliquot - ans.__________

K). meq of Na+ present in the original 50.00 mL seawater sample - ans.__________

L). mg of Na+ present in in the original 50.00 mL seawater sample - ans.__________

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