In this exercise the amount of calcium carbonate (CaCO3) in both an antacid tabl

Question

In this exercise the amount of calcium carbonate (CaCO3) in both an antacid tablet, and an unknown sample must be determined and reported. The chemical analysis method used will be a titration of Ca2+ with the titrant EDTA. EDTA is a ligand that will from a complex ion with Ca2+. EBT is a dye that will be used to indicate the equivalence point of the titration. EBT is also a ligand, and therefore will form a different complex ion with Ca2+ than that of EDTA. In solution the two ligands, EDTA and EBT, compete for the Ca2+ ion. EBT has the property that when it is bonded to Ca2+ it forms a RED complex. When it is not bonded to Ca2+ (or is free from Ca2+) it is BLUE in color. As EDTA is added to the dissolved CaCO3 solution, Ca2+ ions will break the bonds they have initially formed with the EBT dye and make new bonds with the EDTA to form a more stable complex. This process "frees up" EBT molecules in the solution.

Question 1 Each molecule of EDTA makes ____ coordinate covalent bonds to one Ca2+ cation. Enter an integer as an answer. Do not include units.

Question 2 1 mmol of Ca2+ bonds with ____ mmol of EDTA. Enter an integer as an answer. Do not include units.

Question 3 PART 1 When you titrate a solution that contains CaCO3 obtained by dissolving part of an antacid tablet, you want to optimize the accuracy of the buret by dissolving a mass that will consume approximately 25 mL of the titrant EDTA. If the molarity of the EDTA solution is 0.04940, how many mmols of EDTA are there in 25 mL ? Enter a numeric answer only, to THREE significant figures. Do not include units.

Question 4 How many mmols of Ca2+ from an antacid tablet will react with the number of mmols of EDTA that was your answer to Question 3 ? Refer back to Question 1, if necessary. Enter a numeric answer only, to THREE significant figures. Do no include units.

Question 5 The amount of Ca2+ you reported in Question 4 all came from CaCO3, the active ingredient in the antacid tablet. How many mmols of CaCO3 correspond to your answer for Question 4 ? Enter a numeric answer only, to THREE significant figures. Do not include units.

Question 6 How many mg of CaCO3 correspond to your answer for question 5 ? Use 100.1 g/mol as the molar mass of CaCO3. Enter a numeric answer to THREE significant figures. Do not include units.

Question 7 If one antacid tablet contains 500 mg of CaCO3, what percentage of the tablet will provide the mass of Question 6 ? The answer should be around 25 %. Meaning that 1/4 of the antacid tablet contains enough Ca2+ to consume 25 mL of EDTA. Enter a numeric answer only, to TWO significant figures. Do not include the percent sign.

Question 8 You will work alone in this exercise, but you begin by dividing an antacid tablet between yourself and one other person. Weigh one tablet on the analytical balance. Both people must record the total mass in their notebooks. Grind the tablet into a fine uniform powder. Then divide the powder between you. Do this by eye, do not weigh the powder at this point. Your half of the powder will be used for two titrations with the EDTA. If the total mass of the entire solid tablet, before grinding and separating, is 0.7802 g, and you need 1/4 of this amount for your first titration, what mass (g) of powder should you weigh out ? Enter a numeric answer to FOUR significant figures. Do not include units.

Question 9 Weighing by difference you put an actual mass of 0.1974 g of powder into a 250 mL Erlenmeyer flask. Using your 10 mL graduated cylinder add about 5 mL of HCl to the flask to dissolve the powder. When it has completely dissolved, add about 25 mL of water. Add about 10 mL of pH 10 buffer. Add about 1/4 of a spatula tip of EBT. What color is the solution at this point ? Enter a one word answer.

Question 10 The solution described above is now titrated with EDTA. At the equivalence point you will see it change to a pure ________ color. Fill in the bank with a one word answer.

Question 11 You find that your mass of 0.1974 g of powder requires 22.68 mL of EDTA to reach the equivalence point. How many mmol of CaCO3 are in your powder ? The molarity of the EDTA is given above in Question 3 Enter a numeric answer only, to the correct number of significant figures. Do not include units.

Question 12 How many grams of CaCO3 are in your powder ? Enter a numeric answer only, to the correct number of significant figures. Do not include units. Enter Your Answer: Question 13 What is the mass percent of CaCO3 in your powder ? This is, of course, also the mass percent of CaCO3 in the entire antacid tablet. Enter a numeric answer only, to the correct number of significant figures. Do not include units.

Question 14 How many mg of CaCO3 were in the entire solid antacid tablet before grinding and separating ? Mass of tablet is given in Question 8. Enter a numeric answer only, to the correct number of significant figures. Do not include units.

Question 15 You weigh out 129.5 mg of your sample and find it requires 23.58 mL of EDTA to reach the EQ. PT. What is the mass percent of CaCO3 in your sample ? The molarity of the EDTA solution is given in Question 3. Enter a numeric answer only to the correct number of significant figures. Do not include units

Explanation / Answer

Question 1 Each molecule of EDTA makes _6___ coordinate covalent bonds to one Ca2+ cation. Enter an integer as an answer. Do not include units.

EDTA is ethylenediaminetetraacetate and it is usually hexadentate ligand so coordination number is also 6.

Question 2 1 mmol of Ca2+ bonds with _1___ mmol of EDTA. Enter an integer as an answer. Do not include units.

H2Y2-(aq) + Ca2+(aq) CaY2-(aq) + 2 H+(aq)

Here H2Y2-(aq) = EDTA ion
so 1 mole of the EDTA ion reacts with 1 mole of both Ca2+.

Question 3 PART 1 When you titrate a solution that contains CaCO3 obtained by dissolving part of an antacid tablet, you want to optimize the accuracy of the buret by dissolving a mass that will consume approximately 25 mL of the titrant EDTA. If the molarity of the EDTA solution is 0.04940, how many mmols of EDTA are there in 25 mL ? Enter a numeric answer only, to THREE significant figures. Do not include units.

Molarity = Number of moles / volume in L

Number of moles = molarity * volume in L

Number of moles = 0.04940 M * 25 ml *1.0l/1000 ml

Number of moles =1.235*10^-3 moles EDTA

1.0 mol = 1000 milomoles

1.235*10^-3 moles * 1000 milimoles /1.0 moles =1.235 mmoles

Question 4 How many mmols of Ca2+ from an antacid tablet will react with the number of mmols of EDTA that was your answer to Question 3 ? Refer back to Question 1, if necessary. Enter a numeric answer only, to THREE significant figures. Do no include units.

Ca2+ reacts with EDTA in 1:1 molar basis.

1.235 mmoles of Ca2+ from an antacid tablet will react with the number of mmols of EDTA.

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