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You are to determine the percentage purity of a sample of calcium carbonate.
This experiment involves three steps.
In step one, you will react the impure calcium carbonate with an excess of acid.
In step two, you will carry out a titration to find the amount of acid you used in step one.
In step three, you will carry out a second titration to find how much (excess) acid remained following the reaction in step one.
Finally, you will use the values found in the three steps to calculate the percentage purity of the calcium carbonate.
Assume the impurity in the calcium carbonate will not react with acid or alkali.
FA 1 is 0.100 mol dm⁻³ sodium hydroxide, NaOH.
FA 2 is approximately 1 mol dm⁻³ hydrochloric acid, HCl.
FA 3 is FA 2 diluted by a factor of 10, approximately 0.1 mol dm⁻³ hydrochloric acid, HCl.
FA 4 is a sample of impure calcium carbonate, CaCO₃.
methyl orange indicator
Read through the whole method before starting any practical work.
(a) Method
Step 1
● Fill the burette labelled FA 2 with FA 2.
● Run 50.00 cm³ of FA 2 into a 250 cm³ beaker.
● Weigh the tube containing the impure calcium carbonate, FA 4.
● Tip the contents of the tube, in small portions, into the acid to avoid acid spray. Stir the mixture and leave the stirring rod in the beaker.
● Reweigh the tube containing any residue.
● Record the weighings and the mass of FA 4 added in a suitable form below.
Step 2
● Fill the burette labelled FA 1 with FA 1.
● Pipette 25.0 cm³ of FA 3 into a conical flask.
● Add methyl orange indicator.
● Perform a rough titration and record your burette readings in the space below.
The rough titre is ...................... cm³.
● Carry out as many accurate titrations as you think necessary to obtain consistent results.
● Make certain any recorded results show the precision of your practical work.
● Record in a suitable form below all of your burette readings and the volume of FA 1 added in each accurate titration.
Step 3
● Stir the mixture from Step 1 again to ensure that all the solid has dissolved.
● Transfer the solution to the 250 cm³ graduated (volumetric) flask labelled FA 5.
● Rinse the beaker twice with a little distilled water and add the washings to the graduated flask.
● Make the solution up to 250 cm³ with distilled water. Ensure that the contents of the flask are thoroughly mixed.
● Transfer 25.0 cm³ of this solution, FA 5, into a second conical flask using a second pipette.
● Perform a rough titration and record your burette readings in the space below.
The rough titre is ..................... cm³.
● Carry out as many accurate titrations as you think necessary to obtain consistent results.
● Make certain any recorded results show the precision of your practical work.
● Record in a suitable form below all of your burette readings and the volume of FA 1 added in each accurate titration.
(b) (i) From your accurate titration results in Step 2, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
25.0 cm³ of FA 3 required ............. cm³ of FA 1.
(b) (ii) Use your answer from (i) to calculate the number of moles of sodium hydroxide, FA 1, required to react with 25.0 cm³ of FA 3 in Step 2.
moles of NaOH = ......................... mol
(b) (iii) Use your answer from (ii) to determine the number of moles of hydrochloric acid in 25.0 cm³ of FA 3.
moles of HCl in 25.0 cm³ of FA 3 = ......................... mol.
(b) (iv) FA 3 was produced by diluting FA 2. Calculate the number of moles of hydrochloric acid in 50.00 cm³ of FA 2.
moles of HCl in 50.00 cm³ of FA 2 = ......................... mol.
(c) (i) From your accurate titration results in Step 3, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
25.0 cm³ of FA 5 required ............. cm³ of FA 1.
(c) (ii) Use your answer from (i) to calculate the number of moles of sodium hydroxide, FA 1, required to react with 25.0 cm³ of FA 5 in Step 3.
moles of NaOH = ......................... mol
(c) (iii) Use your answer from (ii) to determine the number of moles of hydrochloric acid in 25.0 cm³ of FA 5.
moles of HCl in 25.0 cm³ of FA 5 = ......................... mol
(c) (iv) Use your answer from (iii) to determine the number of moles of hydrochloric acid in 250 cm³ of FA 5.
moles of HCl in 250 cm³ of FA 5 = ......................... mol
(d) (i) Write an equation for the reaction between calcium carbonate and hydrochloric acid.
......................................................................................................................
(d) (ii) Calculate the number of moles of hydrochloric acid that reacted with calcium carbonate in FA 4 using the following expression.
moles of HCl = (b)(iv) − (c)(iv)
= ......................... mol
(d) (iii) Use your answers from (i) and (ii) to calculate the mass of CaCO₃ in FA 4.
[Aᵣ: C, 12.0; O, 16.0; Ca, 40.1]
(If you were unable to answer (d)(ii), you may assume that the number of moles of hydrochloric acid that reacted with calcium carbonate was 0.0351 mol.)
mass of CaCO₃ in FA 4 = ......................... g
(d) (iv) Calculate the percentage purity by mass of the calcium carbonate in FA 4.
The percentage purity by mass of calcium carbonate is ......................... %
(e) (i) What is the maximum error in a single burette reading?
maximum error in a burette reading = ................ cm³
(e) (ii) Calculate the maximum percentage error for one of your accurate titres in Step 3. Show your working.
maximum percentage error = .................. %
(f) A student decided to use a larger mass of FA 4. State and explain whether this alteration will improve the accuracy of the percentage purity obtained.
......................................................................................................................
539
592
539
(a) Compounds FA 5, FA 6 and FA 7 are salts containing the same transition metal but in three different oxidation states. You are provided with solutions of FA 5 and FA 6 and a solid sample of FA 7. Carry out the experiments described below and record your observations in the table.
| test | observations |
|------|--------------|
| (i) To 1 cm depth of FA 5 in a test-tube add about 5 cm depth of dilute sulfuric acid and mix the two solutions. Use this mixture for tests (ii) and (iii). | no observation required |
| (ii) To 1 cm depth of hydrogen peroxide in a test-tube add 10 drops of acidified FA 5 from (i). | |
| (iii) To 1 cm depth of potassium iodide in a test-tube add 1 cm depth of acidified FA 5 from (i). | |
| (iv) To 1 cm depth of FA 6 in a test-tube add 1 cm depth of aqueous sodium hydroxide and allow to stand for a few minutes. | |
| (v) To 1 cm depth of FA 6 in a test-tube add aqueous ammonia until in excess. | |
| (vi) To 1 cm depth of FA 6 in a test-tube add 1 cm depth of FA 5 and allow to stand for a few minutes. | |
| (vii) To 1 cm depth of hydrogen peroxide in a test-tube add a small spatula measure of FA 7. | |
| (viii) Place a small spatula measure of FA 7 in a test-tube and add about 10 drops of concentrated hydrochloric acid with care.
Place the tube in the hot water bath. Test any gas produced with damp litmus paper.
When you have made your observations, fill the test-tube with cold water to stop any further reaction. | |
(b) (i) From your observations in (a) suggest the identity of the transition metal contained in FA 5, FA 6 and FA 7.
The transition metal present in the three compounds is ......................................................
Explain how your observations support your conclusion..............................................................................................................................
(b) (ii) Suggest the type of reaction occurring in (a)(iii).
.......................................................................................................................................................................................
(b) (iii) Give the oxidation state of the transition metal in FA 6.
.....................................................................................
The oxidation state of the transition metal in FA 5 is +7.
Suggest an oxidation state of the transition metal in the product formed in (a)(vi).
.....................................................
(c) Aqueous solutions FA 8 and FA 9 both contain halide ions.
(i) Use the Qualitative Analysis Notes on page 12 to select two reagents which, used together, identify the halide ions in FA 8 and FA 9.
The first reagent is ......................................................
and this is followed by ......................................................
(c) (ii) Use your chosen reagents to carry out tests on FA 8 and FA 9. Record your results in an appropriate form in the space below.
.......................................................................................................................................................................................
(c) (iii) From the results of the tests in (ii), state which halide each solution contains.
FA 8 contains ......................................................
FA 9 contains ......................................................
(c) (iv) Halides can also be identified by reaction with concentrated sulfuric acid. The acid can act as an oxidising agent. State what you would expect to see if concentrated sulfuric acid was added to a solid sample of FA 8 and FA 9. Do not carry out these experiments.
expected observation with FA 8 ......................................................
......................................................
expected observation with FA 9 ......................................................
......................................................
(c) (v) Solutions containing the copper(II) ion react with concentrated hydrochloric acid. To a 1 cm depth of aqueous copper(II) sulfate in a test-tube, add an equal volume of concentrated hydrochloric acid with care.
observation ......................................................
......................................................
509
583
598
