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You will find the relative atomic mass, $A_r$, of magnesium by measuring the volume of hydrogen produced when a known mass of metal reacts with an excess of acid.
$$\text{Mg(s)} + \text{2HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$
FB 1 is 1.00 mol dm$^{-3}$ hydrochloric acid, HCl.
FB 2 is magnesium, Mg.
(a) Method
Read through the whole method before starting any practical work.
• Fill the tub with water to a depth of about 5 cm.
• Weigh the magnesium, FB 2, and note its mass below. If you are using a balance reading to 1 decimal place and the reading with the magnesium is zero, you should record this value.
• Fill the burette to about the 20 cm$^3$ mark with hydrochloric acid, FB 1.
• Add distilled water to reach the 0 cm$^3$ mark on the burette.
• Bend the magnesium strip into a U-shape.
• Place the magnesium in the burette so that it is above the liquid and friction holds it in position. Use a glass rod to push the magnesium about 2 cm into the burette.
• Hold a piece of paper towel over the open end of the burette, invert the burette and immediately place it in the tub of water. Remove the paper towel and clamp the burette as shown in the diagram.
• The liquid level should now be on the scale of the burette. If it is not, open the tap for a moment to allow the level to drop.
• Record the initial reading on the burette. Remember that the scale is now upside down.
• Leave the apparatus so that the acid from the burette diffuses around the magnesium and reacts.
• You should start Question 2 or Question 3 while waiting for the reaction to complete.
• When all the magnesium has reacted, note and record the final reading on the burette.
• Calculate the volume of hydrogen produced.
Results
(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 hydrogen produced.
(Assume that 1 mole of gas occupies 24.0 dm$^3$ under these conditions.)
$\text{moles of H}_2 = \text{......................... mol}$
(ii) Use your answer to (i) and the mass of magnesium used to calculate the $A_r$ of magnesium.
(If you used a balance reading to 1 decimal place, you should assume that the mass of magnesium was 0.04 g correct to 2 decimal places.)
$A_r \text{ of Mg = .........................}$
[2]
(c) (i) Calculate the percentage error in the mass and volume readings in this experiment.
(ii) Suggest a change that could be made to reduce the greater error calculated in (i).
[3]
(d) What would be the effect on the value of the $A_r$ of magnesium calculated if the temperature of the room was much lower than that for your experiment? Explain your answer.
[2]
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In Question 1 you calculated the relative atomic mass, $A_{r}$, of magnesium by measuring the volume of hydrogen produced. The relative atomic mass can also be determined by investigating how much of the hydrochloric acid reacted with the magnesium.
The experiment described in Question 1 was repeated, this time using 0.21 g of magnesium ribbon and 30.0 $cm^{3}$ of 1.00 $mol dm^{-3}$ hydrochloric acid. All the solution left in the burette and tub was kept and water added to make the total volume 250$cm^{3}$. This solution was labelled FB 3.
You will titrate FB 3 using a known concentration of aqueous sodium carbonate to determine how much hydrochloric acid was left over after the reaction with magnesium.
$$Na_2CO_3(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + \ H_2O(l) + \ CO_2(g)$$
FB 3 is the solution of hydrochloric acid described above.
FB 4 is aqueous sodium carbonate containing 2.64 $g dm^{-3}$ $Na_2CO_3$.
bromophenol blue indicator
(a) Method
• Fill the burette with FB 3.
• Pipette 25.0 $cm^{3}$ of FB 4 into a conical flask.
• Add about 10 drops of bromophenol blue 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 consider necessary to obtain consistent results.
• Record, in a suitable form below, all of your burette readings and the volume of FB 3 added in each accurate titration.
• Make certain any recorded results show the precision of your practical work.
(b) From your accurate titration results, obtain a suitable value for the volume of FB 3 to be used in your calculations. Show clearly how you 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) Use the information on page 4 and the Periodic Table on page 12 to calculate the number of moles of sodium carbonate in the 25.0 $cm^{3}$ of FB 4 used in each titration.
moles of $Na_2CO_3$ = .............................. mol
(ii) Use your answer to (i) to calculate the number of moles of hydrochloric acid present in the volume of FB 3 recorded in (b).
$$Na_2CO_3(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + \ H_2O(l) + \ CO_2(g)$$
moles of $HCl$ present = .............................. 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 $HCl$ present in 250 $cm^{3}$ = .............................. mol
(iv) Use the information on page 4 to calculate the number of moles of hydrochloric acid added to the magnesium.
moles of $HCl$ added = .............................. mol
(v) Calculate the number of moles of hydrochloric acid that reacted with the magnesium.
moles of $HCl$ that reacted with the magnesium = .............................. mol
(vi) Use your answer to (v) and the mass of magnesium used to calculate the relative atomic mass, $A_{r}$, of magnesium.
$$Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_{2}(g)$$
$A_{r}$ of Mg = ..............................
(d) A solution of sodium hydroxide was prepared at the same concentration, in $mol dm^{-3}$, as FB 4. A student repeated the titration but replaced FB 4 with this solution of sodium hydroxide.
(i) Explain the effect that replacing FB 4 with this solution of sodium hydroxide would have on the volume of acid, FB 3, needed for the titration.
.......................................................................................................................
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(ii) If the sodium hydroxide had been stored for a long time it would not be suitable for use to find the concentration of the acid.
Suggest why storage for a long time would make the sodium hydroxide unsuitable.
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548
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575
(a) (i) Half fill the 250 cm3 beaker with water. Heat the water to about 80°C and then turn off the Bunsen burner. This is the hot water bath needed in the following tests.
To a 3–4 cm depth of aqueous silver nitrate in a test-tube, add a few drops of aqueous sodium hydroxide to give a grey/brown precipitate. Then add aqueous ammonia dropwise until the precipitate just disappears. This solution is Tollens’ reagent and is needed in the following test.
FB 5, FB 6 and FB 7 are each known to be one of ethanol, propanal and propanone.
Carry out the following tests and complete the table.
[Table_1]
(ii) Use these observations to identify the unknown compounds.
FB 5 is ...................................................
FB 6 is ...................................................
FB 7 is ...................................................
(iii) Choose another reagent that would give a similar result for propanal and propanone but a different result for ethanol.
Do not carry out this test.
reagent ...................................................
result for propanal and propanone ...................................................
result for ethanol ...................................................
(iv) Choose another reagent that would give a similar result for ethanol and propanone but a different result for propanal.
Do not carry out this test.
reagent ...................................................
result for ethanol and propanone ...................................................
result for propanal ...................................................
(b) FB 8 contains one cation and one anion from those listed on pages 10 and 11. You are provided with solid FB 8 and an aqueous solution of FB 8.
(i) To a 1 cm depth of aqueous FB 8 in a test-tube add a 1 cm depth of aqueous sodium hydroxide.
Keep the test-tube and contents for test (ii).
observation ...................................................
(ii) Transfer the contents of the test-tube from test (i) into a boiling tube and heat gently and
carefully.
Allow to cool and keep the boiling tube and contents for test (iii).
observation ...................................................
(iii) Transfer a 1 cm depth of the mixture from test (ii) into a boiling tube and add a 2 cm depth
of dilute hydrochloric acid. Heat gently and carefully.
observation ...................................................
Allow to cool and keep the boiling tube and contents for test (iv).
(iv) To the boiling tube from test (iii) add a piece of aluminium foil. Leave the boiling tube to stand.
observation ...................................................
(v) Place a small spatula measure of solid FB 8 in a hard-glass test-tube and heat it gently at first and then more strongly.
Identify two gases, other than water vapour, that are produced and give your evidence.
identity ...................................................
evidence ...................................................
identity ...................................................
evidence ...................................................
(vi) From your observations in (i) to (v), write the formula of FB 8.
...................................................
(vii) Write the ionic equation for the reaction that is occurring in test (i). Include state symbols.
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