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Metal carbonates react with acid to produce carbon dioxide. You will determine the identity of a Group 2 metal \( M \) in a carbonate of formula \( MCO_3 \) by reacting the carbonate with excess dilute hydrochloric acid and measuring the volume of carbon dioxide produced.
\[ MCO_3(s) + 2HCl(aq) \rightarrow MCl_2(aq) + CO_2(g) + H_2O(l) \]
FA 1 is 50 cm\(^3\) of 4.00 mol dm\(^{-3}\) hydrochloric acid, HCl.
FA 2 is the metal carbonate, \( MCO_3 \).
(a) Method
• Fill the tub with water to a depth of approximately 5 cm.
• Fill the 250 cm\(^3\) measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub.
• Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub.
• Add all the FA 1 into the flask labelled X.
• Check that the bung fits tightly into the neck of flask X, clamp flask X and place the end of the delivery tube into the inverted 250 cm\(^3\) measuring cylinder.
• Weigh the container with FA 2 and record the mass.
• Remove the bung from the neck of the flask. Tip the FA 2, from the container, into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents.
• Replace the flask in the clamp. Leave for several minutes, swirling the flask occasionally.
You may wish to start Question 2 while the gas is being evolved.
• When no more gas is collected, measure and record the final volume of gas in the measuring cylinder.
• Weigh the container, with any residual FA 2, and record the mass.
• Calculate and record the mass of FA 2 added to flask X.
(b) Calculations
(i) Calculate the number of moles of carbon dioxide collected in the measuring cylinder.
[Assume that 1 mol of gas occupies 24.0 dm\(^3\) under these conditions.]
moles of CO\(_2\) = \text{.................................} mol [1]
(ii) Use your answer to (b)(i) and the information on page 2 to calculate the relative atomic mass, \( A_r \), of \( M \).
\( A_r \) of \( M \) = \text{.............................} [3]
(iii) Use your answer to (b)(ii) to identify \( M \).
\( M \) is \text{.............................} [1]
(c) (i) A student suggested that, using the same apparatus, the accuracy of the experiment would be increased if approximately 2 g of \( MCO_3 \) were used to react with the excess hydrochloric acid.
State and explain whether the student was correct.
\text{..................................................................................................................................}
\text{..................................................................................................................................}
\text{..................................................................................................................................} [1]
(ii) Another student suggested that the experiment would be more accurate if the carbon dioxide was collected in a gas syringe rather than over water.
State and explain whether the student was correct.
\text{..................................................................................................................................}
\text{..................................................................................................................................}
\text{..................................................................................................................................} [1]
Results
532
577
535
Question 3:
(a) Method
• Weigh the crucible with its lid and record the mass.
• Add between 1.30g and 1.50g of FA 3 into the crucible. Record the mass of crucible, lid and FA 3.
• Place the crucible on the pipe-clay triangle on the tripod. Put the lid on the crucible and heat gently for approximately 1 minute.
• Use tongs to remove the lid and heat the crucible strongly for approximately 5 minutes. Replace the lid and then leave to cool.
• While the crucible is cooling, begin work on Question 3.
• When cool, reweigh the crucible with its lid and contents. Record the mass.
• Calculate and record the mass of FA 3 placed in the crucible.
• Calculate and record the mass of residue left after heating.
Keep the crucible and its contents for use in Question 3(b).
Results
(b) Calculations
(i) Calculate the number of moles of carbon dioxide produced during heating of FA 3.
moles CO$_2$ = ......................... mol [1]
(ii) Use the mass of FA 3 in (a) and your answer to (b)(i) to calculate the relative atomic mass, A$_r$, of Q and hence identify Q. You should assume complete decomposition of QCO$_3$.
A$_r$ of Q is ..........................
Q is .......................... [4]
(c) Explain why the lid was placed on the crucible when the residue was left to cool.
............................................................ [1]
............................................................
(d) In order to decompose Group 2 carbonates, the solid must be heated strongly. In this experiment QCO$_3$ was heated for a few minutes.
(i) Suggest an improvement to the method used that would ensure that decomposition was complete.
............................................................ [1]
............................................................
(ii) Suggest a chemical test to determine whether the decomposition of QCO$_3$ was complete. State the expected observation if the decomposition was incomplete.
Do not carry out this test.
............................................................ [1]
............................................................
(e) (i) In your calculation in (b) you used the mass of QCO$_3$ and assumed that it was all decomposed during the heating.
Explain what effect incomplete decomposition would have on the calculated value of the A$_r$ of Q.
............................................................ [1]
............................................................
(ii) A student suggested that you could use the mass of the residue, QO, rather than the mass of QCO$_3$ in a calculation to identify Q.
Explain why this method of calculating the A$_r$ of Q is valid.
............................................................ [1]
............................................................
547
535
595
(a) FA 4 and FA 5 are aqueous solutions each containing one anion and one cation.
(i) Carry out the following tests and record your observations. For each test use a 1 cm depth of FA 4 or FA 5 in a test-tube.
[Table]
| test | observations | |------|--------------| | Add a 1 cm depth of dilute hydrochloric acid. Leave to stand. | FA 4 | FA 5 | | Add a 1 cm depth of aqueous copper(II) sulfate. Leave to stand. | | | Add a few drops of aqueous silver nitrate, then
add aqueous ammonia. | | | Add a 1 cm depth of aqueous chlorine, then
add a 1 cm depth of FA 5. | | |
[5]
(ii) From your observations in (a)(i) identify one of the ions present in either FA 4 or FA 5.
Ion present in ................. is ......................... . [1]
(iii) Apart from the reaction with FA 5 suggest a test that could be used to identify the coloured product formed in the reaction between aqueous chlorine and FA 4. You should include the reagent used and the expected observation.
Do not carry out this test.
reagent ..........................................................................................................................
expected observation ................................................................................................. [1]
(b) (i) Place the cooled crucible and residue from Question 2 onto a heatproof mat and add approximately 5 cm³ of water.
Test the solution with litmus papers. Record your observations.
...............................................................................................................................
............................................................................................................................... [1]
(ii) Using QO as the formula of the residue, write the equation for the reaction with water that occurs in (b)(i). Include state symbols.
............................................................................................................................... [1]
(c) In Questions 1 and 2 you identified the Group 2 metals present in MCO₃ and QCO₃.
You will now plan and carry out tests to confirm, or not confirm, the identities of M and Q. Both M and Q are listed in the Qualitative Analysis Notes.
(i) Group 2 carbonates are insoluble in water. In order to test for the cations present (M²⁺ and Q²⁺) they must be in solution.
Name a reagent you could use to prepare solutions of the cations from solid samples of MCO₃ and QCO₃.
.............................................................................................................................. [1]
(ii) You are provided with the following solutions.
FA 6 contains M²⁺(aq).
FA 7 contains Q²⁺(aq).
Choose reagents that could be used to confirm the identity of M and Q. Carry out the tests. Record the tests, observations and conclusions.
..............................................................................................................................
(iii) Do your conclusions confirm your identification of M and Q in Questions 1 and 2? Explain your answer.
..............................................................................................................................
..............................................................................................................................
.............................................................................................................................. [1]
521
510
552
