No questions found
You are provided with the following.
FB 1 is 0.023 mol dm−3 potassium manganate(VII), KMnO4.
FB 2 is aqueous ethanedioic acid, H2C2O4, made by dissolving the hydrated salt, H2C2O4.2H2O.
You are also provided with the following.
1.0 mol dm−3 sulfuric acid, H2SO4
distilled water
You are required to determine the concentration, in g dm−3, of hydrated ethanedioic acid, H2C2O4.2H2O, in FB 2.
Dilution of FB 2
(a) By using a burette, measure between 42.50 cm3 and 43.00 cm3 of FB 2 into the 250 cm3 graduated flask, labelled FB 3.
Record your burette readings and the volume of FB 2 added to the flask in the space below.
Make up the contents of the flask to the 250 cm3 mark with distilled water. Place the stopper in the flask and mix the contents thoroughly by slowly inverting the flask a number of times.
Titration
Fill a second burette with FB 1.
Pipette 25.0 cm3 of FB 3 into a conical flask. Use the measuring cylinder provided to add to the flask 25 cm3 of 1.0 mol dm−3 sulfuric acid and 40 cm3 of distilled water. Put the thermometer in the flask and heat the solution until the temperature is just over 65 °C. Carefully remove the thermometer and place the hot flask under the burette. If the neck of the flask is too hot to hold safely, use a folded paper towel to hold the flask.
Run in 1 cm3 of FB 1. Swirl the flask until the colour of the potassium manganate(VII) has disappeared then continue the titration as normal until a permanent pale pink colour is obtained. This is the end-point.
If a brown colour appears during the titration, reheat the flask to 65 °C. The brown colour should disappear and the titration can be completed as above.
If the brown colour does not disappear on reheating, discard the solution and start the titration again.
Perform a rough (trial) titration and sufficient further titrations to obtain reliable results.
Record your titration results in the space below. Make certain that your recorded results show the precision of your working. [6]
(b) From your titration results obtain a volume of FB 1 to be used in your calculations. Show clearly how you obtained this volume. [1]
Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(c) Calculate how many moles of KMnO4 were run from the burette into the conical flask.
.................... mol of KMnO4 were run from the burette into the conical flask.
Put the correct number of electrons into each of the following half-equations to balance the electrical charges.
MnO4− + 8H+ + ......... e− → Mn2+ + 4H2O
C2O42− → 2CO2 + ......... e−
Calculate how many moles of ethanedioate ions, C2O42−, reacted with the KMnO4 run from the burette.
..................... mol of ethanedioate ions reacted with the KMnO4 run from the burette.
Calculate the concentration, in mol dm−3, of C2O42− in FB 3.
The concentration of C2O42− in FB 3 is ..................... mol dm−3.
Calculate the concentration, in mol dm−3, of C2O42− in FB 2.
The concentration of C2O42− in FB 2 is ..................... mol dm−3.
Calculate the concentration, in g dm−3, of H2C2O4.2H2O in FB 2.
[Ar: H, 1.0; C, 12.0; O, 16.0]
FB 2 contains ..................... g dm−3 H2C2O4.2H2O. [5]
527
503
508
(a) Reaction of FB 4, Na2CO3, with an excess of 3.0 mol dm^-3 hydrochloric acid
Read through the following instructions carefully before starting the experimental work.
• Support the plastic cup in the 250 cm^3 beaker provided.
• Use the measuring cylinder to transfer 50 cm^3 of 3.0 mol dm^-3 hydrochloric acid into the plastic cup.
• Weigh the tube containing FB 4, anhydrous sodium carbonate.
• Measure and record the steady temperature of the acid in the beaker.
• Add the contents of the tube to the acid in three separate lots, taking care that the mixture does not overflow.
• Stir and record the highest temperature obtained.
• Reweigh the tube containing residual FB 4.
Record in an appropriate form below all of your weighings and temperature measurements together with the mass, m_1, of FB 4 added and the temperature rise, ΔT_1.
(b) Calculate the temperature rise per gram of FB 4, Na2CO3, used in the experiment.
$$\frac{\Delta T_1}{m_1} =\text{............................ } ^\circ \text{C g}^{-1}$$ [2]
(c) Calculate the enthalpy change, ΔH_1, for the following reaction.
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
$$ ΔH_1 = -(22.79 \times \frac{\Delta T_1}{m_1}) \text{ kJ mol}^{-1} $$
$$ ΔH_1 = -\text{..................... kJ mol}^{-1} $$
(d) Reaction of FB 5, NaHCO3, with an excess of 3.0 mol dm^-3 hydrochloric acid
50 cm^3 of 3.0 mol dm^-3 hydrochloric acid contains 0.15 mol HCl.
Calculate the mass of NaHCO3 that will react with 0.15 mol HCl.
$$[A_r: C, 12.0; H, 1.0; O, 16.0; Na, 23.0]$$
.......................................................................................................... [1]
(e) The error in reading a -10°C to +110°C thermometer is ±0.5°C.
What is the maximum error when using two temperature measurements to calculate a temperature change?
The maximum error is ± ........................................ °C. [1]
(f) Determine the maximum percentage error in the calculated temperature change when 1.0 g of NaHCO3 is added to 50 cm^3 of 3.0 mol dm^-3 hydrochloric acid.
The maximum error is ± ......................................... %. [1]
(g) Use your answer to (d) and the expected temperature change of -1.5°C g^-1 to select a mass of FB 5, NaHCO3, to use in an experiment with 50 cm^3 of 3.0 mol dm^-3 hydrochloric acid. The mass selected should give an appropriate, measurable, temperature fall.
Note: The hydrochloric acid should be in excess and the percentage error in temperature measurement should be kept to a minimum.
Mass of FB 5 to be used = ......................................... g.
Predicted temperature fall = ......................................... °C. [1]
(h) Read through the instructions before starting any practical work.
• Empty, rinse, and shake dry the plastic cup used in (a).
• Support the plastic cup in the 250 cm^3 beaker provided.
• Use the measuring cylinder to transfer 50 cm^3 of 3.0 mol dm^-3 hydrochloric acid into the plastic cup.
• Weigh the empty tube labelled NaHCO3.
• Add the mass of FB 5 you have selected in (g) to the tube and reweigh.
• Measure and record the steady temperature of the acid in the beaker.
• Add the contents of the tube to the acid in three separate lots, taking care that the mixture does not overflow.
• Stir and record the lowest temperature obtained.
• Reweigh the tube containing residual FB 5.
Record in an appropriate form below all of your weighings and temperature measurements together with the mass, m_2, of FB 5 added and the temperature fall, ΔT_2.
[2]
(i) Calculate the temperature fall per gram of FB 5, NaHCO3, used in the experiment.
$$\frac{\Delta T_2}{m_2} =\text{............................. }^\circ \text{C g}^{-1}$$ [3]
(j) Calculate the enthalpy change, ΔH_2, for the following reaction.
NaHCO3(s) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
$$ ΔH_2 = +(18.06 \times \frac{\Delta T_2}{m_2}) \text{ kJ mol}^{-1} $$
$$ ΔH_2 = +\text{..................... kJ mol}^{-1} $$
(k) It is not possible to measure experimentally the enthalpy change, ΔH_3, for the following reaction as it does not take place in the laboratory.
Na2CO3(s) + CO2(g) + H2O(l) → 2NaHCO3(s)
It is possible, however, to calculate a "theoretical" value of ΔH_3 for this reaction from the results of the experiments you have carried out and a Hess cycle.
$$
2HCl(aq) + Na_2CO_3(s) + H_2O(l) + CO_2(g) \overset{\Delta H_3}{\longrightarrow} 2NaHCO_3(s) + 2HCl(aq) \overset{2\Delta H_2}{\longrightarrow} 2NaCl(aq) + 2CO_2(g) + 2H_2O(l)$$
Derive an equation to link ΔH_1, ΔH_2, and ΔH_3.
Use your equation and the results from (c) and (j) to calculate a value for ΔH_3.
$$ ΔH_3 = \text{............................. kJ mol}^{-1} $$ [2]
(l) Suggest a modification to the experimental method in order to reduce the transfer of heat energy to or from the contents of the plastic cup during the experiment.
..........................................................................................................................
.......................................................................................................................... [1]
559
574
511
FB 6 and FB 7 each contain one of the following sodium halides, NaCl, NaBr, NaI.
(a) Place half of the solid FB 6 provided in a test-tube. Half fill the test-tube with distilled water and shake to dissolve the solid. Label the tube FB 6.
Do the same with FB 7, labelling the tube FB 7.
Keep the remaining solid for (c).
(b) You are to select appropriate reagents from those provided and to perform tests to identify which halide ion is present in FB 6 and which in FB 7.
Retain some of the FB 7 solution for test (d).
In an appropriate form below record the tests performed and the results of those tests.
From the recorded observations the following halides are identified.
FB 6 contains ....................................................
FB 7 contains ....................................................
[4]
(c) Carry out the following tests. [Care: unpleasant fumes may be produced]
| test | observations |
|---|---|
| FB 6 | Place the remaining solid in a clean, dry test-tube and add 5 drops of concentrated sulfuric acid (care: the concentrated acid is very corrosive), then as soon as you have made your observation, half fill the test-tube with distilled water to dissolve the remaining solid and any fumes produced. |
| Transfer 1 cm depth of the resulting solution to a test-tube and add a few drops of starch solution. |
(d) Carry out the following tests.
| test | observations |
|---|---|
| Place 1 cm depth of the solution of FB 7 prepared in (a) in a test-tube. Add 1 cm depth of aqueous bromine, [Care: unpleasant fumes] then, add a few drops of starch solution. |
(e) Use your observations and knowledge of halogen chemistry to explain the reactions in (c) and identify the chemical behaviour of the concentrated sulfuric acid in the reaction.
..............................................................................................................
..............................................................................................................
..............................................................................................................
..............................................................................................................
Use your observations and knowledge of halogen chemistry to explain what happens when the solutions are mixed in (d).
..............................................................................................................
..............................................................................................................
..............................................................................................................
.............................................................................................................. [3]
(f) FB 8 and FB 9 each contain one cation from those listed on page 11.
Carry out the following tests to identify the cation present in each solution.
| test | observations |
|---|---|
| FB 8 | To 1 cm depth of solution in a test-tube, add aqueous sodium hydroxide a little at a time then, add an excess of the reagent to give no more than 4 cm depth of solution in the test-tube. |
| To 1 cm depth of solution in a test-tube, add aqueous ammonia a little at a time then, add an excess of the reagent to give no more than 4 cm depth of solution in the test-tube. |
The cation present in FB 9 is .................................................... [3]
599
534
574
