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You are to determine, by titration, the change in oxidation number of a transition metal ion, M^{2+}, when reacted with acidified potassium manganate(VII).
FA 1 is 0.0200 mol·dm^{-3} potassium manganate(VII), KMnO_4.
FA 2 is 0.0530 mol·dm^{-3} transition metal salt, MSO_4.
FA 3 is 1.0 mol·dm^{-3} sulfuric acid, H_2SO_4.
(a) Method
- Fill the burette with FA 1.
- Pipette 25.0 cm^3 of FA 2 into the conical flask.
- Use the measuring cylinder to add 25 cm^3 of FA 3 into the conical flask.
- Carry out a rough titration and record your burette readings in the space below. Add FA 1 until the contents of the flask turn a permanent pale pink colour.
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 below, in a suitable form, all of your burette readings and the volume of FA 1 added in each accurate titration.[Table_1]
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
25.0 cm^3 of FA 2 required .................. cm^3 of FA 1. [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 potassium manganate(VII) present in the volume of FA 1 calculated in (b).
moles of KMnO_4 = ......................... mol
(ii) Calculate the number of moles of MSO_4 in 25.0 cm^3 of FA 2.
moles of MSO_4 in 25.0 cm^3 = ......................... mol
(iii) Use your answers to (i) and (ii) to calculate the number of moles of MSO_4 that react with 1 mole of KMnO_4.
moles of MSO_4 = ......................... mol
(iv) Two possible equations for the reaction of acidified KMnO_4 with MSO_4 are below.
equation 1: 2KMnO_4 + 10MSO_4 + 8H_2SO_4 \rightarrow K_2SO_4 + 2MnSO_4 + 5M_2(SO_4)_3 + 8H_2O
equation 2: 2KMnO_4 + 5MSO_4 + 8H_2SO_4 \rightarrow K_2SO_4 + 2MnSO_4 + 5M(SO_4)_2 + 8H_2O
State and explain which of these two equations is consistent with your answer to (iii).
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(v) Use your answer to (iv) to state the oxidation number of the transition metal M in the product of the reaction.
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[5]
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518
You will determine the enthalpy change, $\Delta H$, for the reaction between magnesium and dilute sulfuric acid. The equation for the reaction is given below.
$\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)}$
$\text{FA 3}$ is $1.00 \text{mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$.
two different coiled lengths of magnesium ribbon, $\text{Mg}$.
(a) Method
Read through the method before starting any practical work and prepare a table for your results in the space below.
• Weigh the shorter piece of magnesium ribbon and record its mass.
• Support the plastic cup in the $250\, \text{cm}^3$ beaker.
• Use the measuring cylinder to transfer $50\, \text{cm}^3$ of $\text{FA 3}$ into the plastic cup.
• Place the thermometer in the $\text{FA 3}$ in the plastic cup and record the initial temperature.
• Add the shorter piece of magnesium ribbon into the plastic cup. Ensure that all of the magnesium is in contact with the acid. (Care: acid spray may occur.)
• Stir the mixture and record the maximum temperature.
• Empty and rinse the plastic cup. Shake out any excess water.
• Repeat the experiment using the longer piece of magnesium ribbon and record all your data.
Results
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Show by calculation that the sulfuric acid, $\text{FA 3}$, was used in excess in both experiments.
$(A_{\text{r}}: \text{Mg}, 24.3)$
(ii) State an observation which confirms that the sulfuric acid, $\text{FA 3}$, was in excess.
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(iii) Calculate the heat energy produced when the shorter piece of magnesium was added to $\text{FA 3}$.
(Assume that $4.3\, \text{J}$ of heat energy changes the temperature of $1.0\, \text{cm}^3$ of solution by $1.0^\circ \text{C}$.)
\text{heat energy produced} = ................ J
(iv) Calculate the enthalpy change, in $\text{kJ mol}^{-1}$, for the reaction between the shorter piece of magnesium and the sulfuric acid.
\text{enthalpy change} = ...... ................ \text{kJ mol}^{-1}
(sign) (value)
(v) Calculate the heat energy produced when the longer piece of magnesium was added to $\text{FA 3}$.
(Assume that $4.3\, \text{J}$ of heat energy changes the temperature of $1.0\, \text{cm}^3$ of solution by $1.0^\circ \text{C}$.)
\text{heat energy produced} = ................ J
(vi) Calculate the enthalpy change, in $\text{kJ mol}^{-1}$, for the reaction between the longer piece of magnesium and the sulfuric acid.
\text{enthalpy change} = ...... ................ \text{kJ mol}^{-1}
(sign) (value)
(c) (i) What is the maximum error in a reading of the thermometer used in this experiment?
maximum error = ......... $^\circ \text{C}$.
(ii) Which of your temperature changes has the higher percentage error?
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(iii) Calculate this maximum percentage error.
maximum percentage error in the temperature change = ................ %
(d) Apart from errors due to heat loss and thermometer readings, suggest another significant source of error in this experiment. State what improvement could be made to the procedure to reduce this error.
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508
540
531
(a) FA 4, FA 5 and FA 6 are solutions, each containing one transition metal ion. One of the solutions also contains the ammonium ion. All the cations present are listed in the Qualitative Analysis Notes on page 10.
(i) Carry out the following tests on the three solutions.
[Table Placeholder]
To a 1 cm depth of FA 4 in a test-tube, add FA 1, aqueous potassium manganate(VII), dropwise.
To a 1 cm depth of FA 5 in a test-tube, add FA 1, aqueous potassium manganate(VII), dropwise.
To a 1 cm depth of FA 6 in a test-tube, add FA 1, aqueous potassium manganate(VII), dropwise.
(ii) State which solution(s) contain ions which have been oxidised.
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(b) (i) Select a reagent or reagents to identify all the cations present in the three solutions.
reagent(s) .................................................................................................................
Carry out experiments using your reagent(s) on each of FA 4, FA 5 and FA 6 and record your observations in a suitable form in the space below.
(ii) Use your observations to identify the cations present in the three solutions.
FA 4 contains ...................................................
FA 5 contains ...................................................
FA 6 contains ...................................................
(c) Each of the solutions FA 4, FA 5 and FA 6 contains either a chloride or a sulfate ion.
(i) Choose a reagent or reagents to identify which solution(s) contain chloride ions.
reagent(s) .................................................................................................................
Use your reagent(s) to carry out a test on each of FA 4, FA 5 and FA 6 and record your results in the space below.
(ii) State which solution(s) contain a chloride ion.
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531
543
528
