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In this experiment you will determine the identity of the Group 2 metal, X, in the carbonate, XCO_3. To do this you will react a known mass of XCO_3 with excess hydrochloric acid, HCl, and measure the mass of carbon dioxide that is given off.
FA 1 is XCO_3.
FA 2 is hydrochloric acid, HCl.
(a) Method
• Weigh the stoppered tube containing FA 1 and record its mass.
• Use the measuring cylinder to transfer 25 cm³ of FA 2 into the 250 cm³ beaker.
• Weigh the beaker containing the acid and record the mass.
• Carefully add all the sample of FA 1 to the acid in the beaker.
• Stir the mixture until there is no further reaction.
• Reweigh the beaker and its contents and record the mass.
KEEP THE CONTENTS OF THE BEAKER FOR USE IN QUESTION 2.
• Reweigh the stoppered tube containing any residual FA 1 and record its mass.
• Calculate the mass of FA 1 added to the acid and record this value.
• Calculate the mass of carbon dioxide given off and record this value.
(b) 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 carbon dioxide given off when XCO_3 reacted with the acid. Use the data in the Periodic Table on page 16.
moles of CO_2 = ................ mol
(ii) Write the equation for the reaction of FA 1, XCO_3, with hydrochloric acid, HCl. Include state symbols.
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(iii) Use your answers to (i) and (ii) to calculate the number of moles of XCO_3 that were added to the acid.
moles of XCO_3 = .......................... mol
(iv) Use your answer to (iii) to calculate the relative atomic mass, A_r, of X. Identify X.
A_r of X = .............................
X is .............................
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(c) One of the sources of error in this experiment is that it is very difficult to reduce acid spraying out of the beaker when the metal carbonate is added to the acid.
(i) Explain what effect this acid spray would have on the value you calculated for the relative atomic mass, A_r, of X.
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(ii) Why is a small amount of acid spray not likely to cause an error in the identification of X?
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(iii) How could you minimise acid spraying out of the beaker?
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In this experiment you will determine the concentration of the hydrochloric acid, FA 2, used in Question 1. You will first dilute the reaction mixture that you prepared in Question 1 and then titrate this diluted solution against sodium hydroxide, NaOH.
$$\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$$
FA 3 is 0.0400 mol dm$^{-3}$ sodium hydroxide, NaOH.
methyl orange indicator
(a) Method
Dilution
• Transfer all the reaction mixture that you prepared in 1(a) from the 250 cm$^3$ beaker to the 250 cm$^3$ volumetric flask.
• Rinse the beaker with a little distilled water and add these washings to the volumetric flask.
• Fill the volumetric flask to the line with distilled water. Stopper the flask and shake it to ensure thorough mixing.
• Label this solution FA 4.
Titration
• Fill the burette with FA 4.
• Use a pipette to transfer 25.0 cm$^3$ of FA 3 into a conical flask.
• Add a few drops of methyl orange.
• 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 FA 4 added in each accurate titration.
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(b) From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value.
25.0 cm$^3$ of FA 3 required .................. cm$^3$ of FA 4.
(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, NaOH, present in 25.0 cm$^3$ of FA 3.
moles of NaOH = .................. mol
(ii) Calculate the number of moles of hydrochloric acid, HCl, present in 250 cm$^3$ of FA 4.
moles of HCl in 250 cm$^3$ of FA 4 = .................. mol
(iii) Use your answers to 1(b)(i) and 1(b)(ii) to calculate the number of moles of HCl that reacted with FA 1 in the experiment you carried out in Question 1.
moles of HCl that reacted with FA 1 = .................. mol
(iv) Use your answers to 2(c)(ii) and 2(c)(iii) to calculate the concentration of FA 2.
concentration of FA 2 = .................. mol dm$^{-3}
(d) (i) One of the sources of error in determining the concentration of FA 2 involves measuring volumes of solutions in both Questions 1 and 2. State which volume of solution that you have measured has the greatest percentage error. How could you have reduced this error? ......................................................................................................................................... ......................................................................................................................................... .........................................................................................................................................
(ii) A student suggested that a greater mass of XCO$_3$ should be used so that the average titre calculated in 2(b) would be a greater volume. Explain whether you agree with the student that this would lead to a greater volume for the average titre. ......................................................................................................................................... ......................................................................................................................................... .........................................................................................................................................
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(a) (i) Carry out the following test and record your observations.
[Table_1: test, observations: Place a small spatula measure of FA 5 in a hard-glass test-tube and heat strongly. Test any gases that are given off.]
(ii) Identify one of the cations in FA 5.
One of the cations in FA 5 is ..................... .
(b) Place the remaining sample of FA 5 in the 100 $\text{cm}^3$ beaker. Half fill the beaker with distilled water and stir until FA 5 has fully dissolved. This may take some time. You will use this solution in the remaining tests.
(i) Select reagents to identify the other cation present in FA 5. Carry out tests using these reagents and record your results in the space below. Identify the cation.
The other cation in FA 5 is ..................... .
(ii) Carry out the following tests and record your observations. Identify one of the anions in FA 5.
[Table_2: test, observations: To a 1 cm depth of the solution of FA 5 in a test-tube add aqueous barium chloride or aqueous barium nitrate, then add dilute hydrochloric acid.]
One of the anions in FA 5 is ..................... .
(iii) The remaining ion is a halide.
Select a pair of reagents which can be used to identify the halide present. Carry out a test using these reagents and record your observations below. Suggest the identity of the halide anion present in FA 5. Explain why this test is not conclusive in this particular case.
The other anion in FA 5 is ..................... .
(c) Suggest the formulae of the two salts that could have been mixed to make FA 5.
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(d) FA 6 and FA 7 are different organic liquids. Their possible identities are listed below.
- 2-methylpropan-2-ol
- propanal
- propanone
Half fill the 250 $\text{cm}^3$ beaker with water and heat to about 50$^{\circ}$C. You will use this as a hot water bath.
Turn off the Bunsen burner.
Carry out the following tests and record your observations.
[Table_3: test, observations: To a 1 cm depth of FA 6 in a test-tube, add a few drops of acidified potassium manganate(VII). If no reaction is seen, warm the solution in the hot water bath.]
[Table_4: test, observations: To a 1 cm depth of FA 7 in a test-tube, add a few drops of acidified potassium manganate(VII). If no reaction is seen, warm the solution in the hot water bath.]
Suggest the identity of FA 6 and FA 7 with an explanation.
FA 6 ......................................................................................................................
FA 7 ......................................................................................................................
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