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You are to determine the percentage of calcium carbonate in a sample of crushed limestone. You will first react a known mass of the crushed limestone in a known amount of hydrochloric acid, HCl(aq), to make a solution. You may assume that only the calcium carbonate present in the sample will react with the acid.
$$ ext{CaCO}_3(s) + 2 ext{HCl}(aq) \rightarrow ext{CaCl}_2(aq) + ext{H}_2 ext{O}(l) + ext{CO}_2(g)$$
The amount of acid that did not react with the carbonate is then found by a titration using sodium hydroxide. You may assume that no compounds present in the limestone will react with the sodium hydroxide.
$$ ext{NaOH}(aq) + ext{HCl}(aq) \rightarrow ext{NaCl}(aq) + ext{H}_2 ext{O}(l)$$
FB 1 is crushed limestone, impure calcium carbonate.
FB 2 is 2.0 mol dm$^{-3}$ hydrochloric acid, HCl.
FB 4 is 0.20 mol dm$^{-3}$ sodium hydroxide, NaOH.
methyl orange indicator
(a) Method
Read through the method before starting any practical work.
Making the solution
- Weigh the container with the limestone, FB 1, and record the mass below.
- Tip all the solid FB 1 into a 250 cm$^{3}$ beaker.
- Reweigh the container and record the mass.
- Fill the burette with FB 2.
- Slowly run between 47.5 and 48.5 cm$^{3}$ of FB 2 into the beaker containing FB 1.
- Record, in the space below, both your burette readings and the volume of FB 2 added.
- Stir the mixture carefully until all the solid has reacted.
- Transfer the contents of the beaker into the volumetric flask.
- Rinse the beaker with distilled water and add it to the volumetric flask. Make the solution up to 250 cm$^{3}$ with distilled water and mix thoroughly. This is solution FB 3.
Titration
- Empty and rinse the burette with distilled water.
- Fill the burette with FB 3 from the volumetric flask.
- Pipette 25.0 cm$^{3}$ of FB 4 into a conical flask.
- Add a few drops of methyl orange indicator.
- Perform a rough titration and record your burette readings in the space below.
The rough titre is ...................... cm$^{3}$.
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 FB 3 added in each accurate titration.
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
25.0 cm$^{3}$ of FB 4 required ............... cm$^{3}$ of FB 3.
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate the number of moles of sodium hydroxide present in 25.0 cm$^{3}$ of FB 4.
moles of NaOH = ...................... mol
(ii) Hence state the number of moles of hydrochloric acid present in the volume of FB 3 calculated in (b).
moles of HCl = ...................... mol
(iii) Use your answer to (ii) to calculate the number of moles of hydrochloric acid present in 250 cm$^{3}$ of FB 3.
moles of HCl in 250 cm$^{3}$ of FB 3 = ...................... mol
(iv) Calculate the number of moles of hydrochloric acid, FB 2, added to FB 1 in (a).
moles of HCl added to FB 1 = ...................... mol
(v) Use your answers to (iii) and (iv) to calculate the number of moles of hydrochloric acid that reacted with the calcium carbonate in FB 1.
moles of HCl reacted with CaCO$_3$ = ...................... mol
(vi) Calculate the number of moles of calcium carbonate present in your sample of FB 1.
moles of CaCO$_3$ = ...................... mol
(vii) From your answer to (vi) and the mass of FB 1 used in (a), calculate the percentage by mass of calcium carbonate in the limestone.
[$A_r$: C, 12.0; O, 16.0; Ca, 40.1]
percentage of calcium carbonate = ...................... %
(d) (i) The maximum error in a single burette reading is ±0.05 cm$^{3}$. Student X, carrying out this experiment, recorded that 48.50 cm$^{3}$ of FB 2 was added to FB 1.
What are the smallest and largest possible volumes of FB 2 that were added?
smallest volume used = ................... cm$^{3}$
largest volume used = ................... cm$^{3}$
(ii) Student Y used an identical mass of FB 1 but added 47.70 cm$^{3}$ of FB 2. How would the value obtained in (b) by student X compare with the value obtained by student Y? Explain your answer.
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You are to determine the percentage by mass of calcium carbonate in another sample of the limestone by thermal decomposition. You may assume that none of the other compounds in the limestone sample is affected by heating. The equation for the reaction that occurs is given below.
$$\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$$
FB 5 is crushed limestone, impure calcium carbonate.
(a) Method
Read through the method before starting any practical work and prepare a table for your results in the space below.
• Weigh the empty crucible and record the mass in your table.
• Transfer all the FB 5 into the crucible.
• Weigh the crucible with FB 5 and record the mass.
• Place the crucible on the pipe-clay triangle.
• Heat the crucible gently for about one minute and then strongly for three minutes.
• Remove the Bunsen burner and allow the crucible to cool.
• While the crucible is cooling start working on another question.
• Reweigh the cooled crucible with contents and record the mass.
• Reheat the crucible strongly for three minutes, cool and reweigh. Record the mass.
• Record the mass of FB 5 used and the mass of solid remaining after the second heating.
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) From your results in (a), calculate the total mass of carbon dioxide lost on heating FB 5.
mass of CO\(_2\) lost = ............... g
(ii) Use your answer to (i) to calculate the mass of calcium carbonate present in the sample of FB 5 heated.
[A\(_r\): C, 12.0; O, 16.0; Ca, 40.1]
mass of CaCO\(_3\) = ............... g
(iii) Calculate the percentage by mass of calcium carbonate in the limestone.
percentage of calcium carbonate = ............... %
(c) FB 5 and FB 1 are samples of the same limestone. You have determined the percentage of calcium carbonate in both Questions 1 and 2 using two different procedures.
(i) Which procedure is the less accurate? Explain your answer.
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(ii) Suggest a change to the less accurate practical procedure that would improve the accuracy and explain your answer.
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(a) A different sample of limestone was reacted with dilute nitric acid to give solution FB 6. This sample of limestone contained calcium carbonate, $\text{CaCO}_3$, and one other salt. This additional salt contains a single cation and a single anion from those listed on pages 11 and 12. By carrying out the following tests you will be able to suggest identities of the additional ions.
[Table_1]
(i) To a 1 cm depth of FB 6 in a test-tube add aqueous ammonia.
(ii) To a 1 cm depth of FB 6 in a test-tube add a 1 cm depth of aqueous silver nitrate.
(iii) To a 1 cm depth of FB 6 in a test-tube add a 1 cm depth of aqueous barium chloride or barium nitrate.
(iv) Suggest all possible identities for the ions present in FB 6, apart from $\text{Ca}^{2+}$ and $\text{NO}_3^-$.
(v) Select a reagent to use in a further test on FB 6 to confirm that one of the cations you have listed in (iv) is not present in FB 6. Carry out your test and complete the table.
[Table_2]
(b) You are provided with a solid, FB 7. By carrying out the following tests you will be able to identify three of the ions present in FB 7.
[Table_3]
(i) Place a spatula measure of FB 7 in a hard-glass test-tube. Heat gently at first, then heat more strongly until no further change is seen, then allow the tube to cool.
(ii) Place a spatula measure of FB 7 in a test-tube. Add about a 5 cm depth of dilute nitric acid. You will use the solution formed for tests (iii) to (v).
(iii) To a 1 cm depth of the solution in a test-tube add a few drops of aqueous potassium manganate(VII), then add a few drops of starch solution.
(iv) To a 1 cm depth of the solution in a test-tube add a few drops of aqueous silver nitrate.
(v) To a 1 cm depth of the solution in a test-tube add aqueous ammonia.
Use the Qualitative Analysis Notes on pages 11 and 12 to identify three of the ions present.
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