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In this experiment you will determine the relative atomic mass, $A_r$, of magnesium by a titration method.
FB 1 is 2.00 mol dm$^{-3}$ hydrochloric acid, HCl.
FB 3 is 0.120 mol dm$^{-3}$ sodium hydroxide, NaOH.
magnesium ribbon
bromophenol blue indicator
(a) Method
Reaction of magnesium with FB 1
● Pipette 25.0 cm$^3$ of FB 1 into the 250 cm$^3$ beaker.
● Weigh the strip of magnesium ribbon and record its mass.
mass of magnesium = .................. g
● Coil the strip of magnesium ribbon loosely and then add it to the FB 1 in the beaker.
● Stir the mixture occasionally and wait until the reaction has finished.
Dilution of the excess acid
● Transfer all the solution from the beaker into the volumetric flask.
● Make the solution up to the mark using distilled water.
● Shake the flask to mix the solution before using it for your titrations.
● Label this solution of hydrochloric acid FB 2.
Titration
● Fill the burette with FB 2.
● Rinse the pipette out thoroughly. Then pipette 25.0 cm$^3$ of FB 3 into a conical flask.
● Add several drops of bromophenol blue indicator.
● Perform a rough titration, by running the solution from the burette into the conical flask until the mixture just becomes yellow.
● 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 sure 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 2 added in each accurate titration.
(b) From your accurate titration results, obtain a suitable value for the volume of FB 2 to be used in your calculations.
Show clearly how you have obtained this value.
25.0 cm$^3$ of FB 3 required .................. cm$^3$ of FB 2.
(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 solution FB 3.
moles of NaOH = .................. mol
(ii) Give the equation for the reaction of hydrochloric acid, HCl, with sodium hydroxide, NaOH.
State symbols are not required.
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Deduce the number of moles of hydrochloric acid in the volume of FB 2 you calculated in (b).
moles of HCl = .................. mol
(iii) Calculate the number of moles of hydrochloric acid in 250 cm$^3$ of FB 2.
moles of HCl in 250 cm$^3$ of FB 2 = .................. mol
(iv) Calculate the number of moles of hydrochloric acid in 25.0 cm$^3$ of FB 1.
moles of HCl in 25.0 cm$^3$ of FB 1 = .................. mol
(v) In (a), you reacted 25.0 cm$^3$ of FB 1 with your weighed piece of magnesium. After the reaction, the unreacted hydrochloric acid was used to prepare 250 cm$^3$ of FB 2.
Use your answers to (iii) and (iv) to calculate the number of moles of hydrochloric acid that reacted with the magnesium ribbon.
moles of HCl reacting with Mg = .................. mol
(vi) Complete the equation below, for the reaction of magnesium with hydrochloric acid.
State symbols are required.
$$\text{Mg + HCl } \rightarrow \text{ MgCl}_2 + ..................$$
Use your answer to (v) to calculate the number of moles of magnesium used.
moles of Mg = .................. mol
(vii) Use your answer to (vi) to calculate the relative atomic mass, $A_r$, of magnesium.
$A_r$ of Mg = ..................
(d) (i) State one observation that proves that the hydrochloric acid in FB 1 was in excess for the reaction with the magnesium ribbon.
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(ii) A student carried out exactly the same experiment but used 1.00 g of magnesium ribbon.
State and explain why the student’s experiment could not be used to determine the value for the $A_r$ of magnesium.
Include a calculation in your answer.
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585
582
518
In this experiment you will determine the relative atomic mass of magnesium by thermal decomposition of hydrated magnesium sulfate.
$$ \text{MgSO}_4 . 7\text{H}_2\text{O(s)} \to \text{MgSO}_4(s) + 7\text{H}_2\text{O(g)} $$
FB 4 is hydrated magnesium sulfate, $\text{MgSO}_4 . 7\text{H}_2\text{O}$.
(a) Method
Record all your weighings in the space below.
• Weigh the crucible with its lid.
• Transfer all FB 4 into the crucible.
• Weigh the crucible, lid and FB 4.
• Place the crucible on the pipe-clay triangle.
• Heat the crucible gently with the lid on, for about one minute.
• Then heat the crucible strongly, without the lid, for a further four minutes.
• Leave the crucible and its contents to cool with the lid on, for several minutes.
• While the crucible is cooling, begin work on Question 3.
• When the crucible has cooled, weigh it, with the lid and contents.
• Calculate and record the mass of anhydrous magnesium sulfate produced and the mass of water lost.
(b) Calculations
(i) Calculate the number of moles of water lost during heating.
(Use the data in the Periodic Table on page 12.) moles of H2O = .................. mol
(ii) Use the equation above and your answer to (i) to calculate the number of moles of anhydrous magnesium sulfate produced. moles of MgSO4 = .................. mol
(iii) Use your weighings and your answer to (ii) to calculate the relative formula mass, $\text{M}_r$, of anhydrous magnesium sulfate. $\text{M}_r$ of MgSO4 = ..................
(iv) From your answer to (iii), calculate the relative atomic mass, $\text{A}_r$, of magnesium. $\text{A}_r$ of Mg = ..................
[4]
(c)
(i) How could the experiment be improved to ensure that the magnesium sulfate had been completely dehydrated?
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(ii) Why is the lid put on the crucible during cooling?
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526
537
593
(a) FB 5 is a solution containing one cation and one anion. Carry out test-tube tests to find out whether the cation in FB 5 is magnesium and whether the anion is sulfate.
• State what reagents you used.
• Record the observations you made in a table.
• State your conclusions about which ions are present.
(b) FB 6 is a salt containing one cation and one anion from those listed on pages 10 and 11.
(i) Place a few crystals of FB 6 in a hard-glass test-tube. Heat gently at first and then strongly. Leave the test-tube and its contents to cool. Record all your observations below.
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(ii) Dissolve the remainder of FB 6 in about 20 cm3 of distilled water in a boiling tube for use in the following tests.
[Table_1]
(iii) Deduce the formula of the salt in FB 6.
Formula is ..........................................
574
593
543
