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You are provided with a solution of an organic acid which is known to be one of the following.
methanoic acid, HCOOH
propanoic acid, C_2H_5COOH
pent-2-enoic acid, CH_3CH_2CH=CHCOOH
The solution was made by dissolving 1.85 g of acid in 250 cm^3 of solution.
You are to suggest the identity of the acid by finding its relative molecular mass, $M_r$, using a titration method.
FA 1 is the solution of the unknown organic acid.
FA 2 is 0.100 mol dm^{-3} sodium hydroxide, NaOH.
phenolphthalein indicator
(a) Method
- Fill the burette with FA 2.
- Pipette 25.0 cm^3 of FA 1 into a conical flask.
- Titrate FA 1 with FA 2 using phenolphthalein as 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 FA 2 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 obtained this value.
25.0 cm^3 of FA 1 required .............. cm^3 of FA 2 [1]
(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 in the volume of FA 2 you have calculated in (b).
moles of NaOH = ................. mol
(ii) One mole of any of the organic acids reacts with one mole of sodium hydroxide. Calculate the concentration, in mol dm^{-3}, of the acid in FA 1.
concentration of the acid in FA 1 = ................. mol dm^{-3}
(iii) Calculate the concentration, in g dm^{-3}, of the acid used to make solution FA 1.
concentration of the acid in FA 1 = ................. g dm^{-3}
(iv) Using your answers to (ii) and (iii), calculate the relative molecular mass, $M_r$, of the acid in FA 1.
$M_r$ of the acid = ..................
(v) Suggest which of the acids, methanoic, propanoic or pent-2-enoic acid, is present in FA 1.
Acid present is ............................................ .
(vi) Suggest a test that could be carried out to distinguish pent-2-enoic acid from methanoic acid and propanoic acid. Give the expected result of your test.
test ......................................................................................
expected result ...................................................................... [5]
503
532
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The formula of hydrated iron(II) sulfate is FeSO$_4$.xH$_2$O where x shows the number of moles of water of crystallisation. The value of x can be found by heating solid hydrated iron(II) sulfate to remove the water of crystallisation.
FA 3 is hydrated iron(II) sulfate, FeSO$_4$.xH$_2$O.
(a) Method
Record all weighings, in an appropriate form, in the space below.
• Weigh and record the mass of the empty crucible.
• Tip the contents of the tube labelled FA 3 into the weighed crucible. Reweigh and record the mass of the crucible and FA 3.
• Use a pipe-clay triangle to support the crucible and contents on a tripod.
• Heat gently for about three minutes.
• Leave the crucible to cool for approximately five minutes.
While you are waiting for the crucible to cool, start work on Question 3.
• When cool, reweigh the crucible with the residue.
• Reheat gently for three minutes, cool and reweigh the crucible until you are satisfied that all the water of crystallisation has been lost. It should not be necessary to reheat the crucible more than three times. [6]
(b) (i) Calculate the mass of water lost and the mass of anhydrous iron(II) sulfate that remains after the heating process.
mass of water lost = ............... g
mass of anhydrous iron(II) sulfate = ............... g
(ii) Determine the value of x in the formula of hydrated iron(II) sulfate, FeSO$_4$.xH$_2$O.
($A_r$: H, 1.0; O, 16.0; S, 32.1; Fe, 55.8)
value of x = ......................
[3]
(c) A group of students carried out this practical and made their measurements correctly. The students calculated a value of 9 for x. The textbook value of x is less than 9.
(i) Suggest an error in the practical procedure of the experiment that could account for this result and explain why this gives a value of x that is too high.
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(ii) Suggest a modification that could be made to the experimental procedure to reduce this error. Explain why this modification should give an answer for x that is closer to the textbook value.
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[3]
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558
526
You are provided with a solid FA 4. FA 4 is a mixture that contains two cations and two anions.
(i) Place a spatula measure of FA 4 in a hard-glass test-tube. Heat the solid and identify the gas given off. Record all your observations.
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(ii) To a spatula measure of FA 4 in a test-tube, add a 1 cm depth of dilute nitric acid. Record your observations.
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(iii) To a spatula measure of FA 4 in a test-tube, add approximately a 2 cm depth of distilled water to make a solution. Divide the solution into two portions.
To the first portion, add a 1 cm depth of aqueous sodium hydroxide. Record your observations.
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To the second portion, add a few drops of aqueous silver nitrate, then add a 1 cm depth of dilute nitric acid. Record your observations.
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(iv) Use your results from (i) to (iii) to identify two anions and one cation that are present in FA 4.
anions present ............................................... and ...............................................
cation present ...............................................
(v) What further test could be carried out on FA 4 to confirm the presence of the cation you suggested in (iv)? You should state the reagent to be used and the expected result.
Do not carry out this test.
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(vi) To a spatula measure of FA 4 in a test-tube, add a 1 cm depth of distilled water to make a solution. To this solution, add a few drops of aqueous barium chloride or barium nitrate.
Describe the appearance of the precipitate formed and state its identity.
appearance of precipitate ...............................................
identity of precipitate ...............................................
FA 5, FA 6, FA 7 and FA 8 are aqueous solutions each containing one of the ions Al 3+, Ca 2+, Zn 2+ and Pb 2+.
(i) Carry out the following tests. Record your observations in the spaces provided in the table.
[Table_1]
To a 1 cm depth of solution in a test-tube, add a few drops of aqueous sodium hydroxide, then
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add excess aqueous sodium hydroxide.
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To a 1 cm depth of solution in a test-tube, add a few drops of aqueous ammonia, then
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add excess aqueous ammonia.
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To a 1 cm depth of solution in a test-tube, add a 1 cm depth of aqueous potassium iodide.
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(ii) Use the Qualitative Analysis Notes on page 10 to identify the cation present in each of the solutions.
FA 5 is .................. , FA 6 is .................. , FA 7 is .................. , FA 8 is ..................
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