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When hydrated barium chloride, BaCl$_2$.xH$_2$O, dissolves in water, Ba$^{2+}$(aq) and Cl$^{-}$(aq) ions are formed.
The concentration of chloride ions in solution can be determined by titration with aqueous silver nitrate of known concentration.
Ag$^{+}$(aq) + Cl$^{-}$(aq) → AgCl(s)
The indicator for the reaction is aqueous potassium chromate(VI), K$_2$CrO$_4$(aq). At the endpoint of the titration, it forms a red precipitate in the presence of excess silver ions.
(a) The solubilities, in g dm$^{-3}$, of different ionic compounds at 20°C are given in the table below.
[Table_1]
With reference to these data, where relevant, answer the following questions.
(i) Name the red precipitate and give an equation for its formation.
name: ......................................................
equation: ...................................................
[2]
Sulfuric acid must be added to the solution to prevent the Ba$^{2+}$(aq) ions from interfering with the action of the potassium chromate(VI) indicator.
(ii) How would Ba$^{2+}$(aq) ions interfere with the action of this indicator?
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[1]
(iii) How does the addition of sulfuric acid prevent Ba$^{2+}$(aq) ions from interfering with the action of this indicator?
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[1]
(b) In an initial rough titration, excess silver nitrate solution is added so that the endpoint is exceeded.
Draw a sketch graph to show how the mass of silver chloride varies with the volume of silver nitrate added.
Label both axes.
[2]
(c) You are to plan a titration experiment to determine the value of x in BaCl$_2$.xH$_2$O.
You are provided with the following materials.
3.00 g of hydrated barium chloride, BaCl$_2$.xH$_2$O
0.050 mol dm$^{-3}$ aqueous silver nitrate
1.0 mol dm$^{-3}$ potassium chromate(VI) solution
1.0 mol dm$^{-3}$ sulfuric acid
(i) Name three pieces of volumetric apparatus you would use, with their capacities in cm$^3$.
1 ...............................................................
2 ...............................................................
3 ...............................................................
[2]
(ii) Describe how you would make a solution of barium chloride that is suitable for use in your titration.
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[2]
(iii) A known volume of barium chloride solution is transferred to a conical flask.
In what order should the other three solutions then be added to the flask?
first ...............................................................
second .............................................................
third ...............................................................
[1]
(iv) How would you ensure that your titration result is reliable?
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[1]
(v) In another experiment, a student dissolved 3.13 g of hydrated barium chloride, BaCl$_2$.xH$_2$O, in distilled water to give 1.00 dm$^3$ of solution.
It was calculated that the concentration of Ba$^{2+}$(aq) ions was 0.0128 mol dm$^{-3}$.
Determine the value of x in BaCl$_2$.xH$_2$O.
[A$_r$: Ba, 137.3; Cl, 35.5; H, 1.0; O, 16.0]
x = ...............................................................
[2]
(d) The following information gives some of the hazards associated with the chemicals used in the procedure.
[Table_2]
Identify one hazard that must be considered when planning the experiment and describe a precaution, other than eye protection, that should be taken to keep risks from this hazard to a minimum.
hazard: ...............................................................
precaution: ............................................................
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[1]
547
505
593
(a) The experimentally determined values of optical rotation during the hydrolysis of sucrose at 298 K are recorded below.
Process the results to allow you to plot a graph of $\log_{10}(\alpha - \alpha_{\text{final}})$ against time, $t$.
Calculate $(\alpha - \alpha_{\text{final}})$ and record it to 1 decimal place.
Calculate $\log_{10}(\alpha - \alpha_{\text{final}})$ and record it to 2 decimal places.
| time / s | optical rotation, $\alpha$ | $(\alpha - \alpha_{\text{final}})$ | $\log_{10}(\alpha - \alpha_{\text{final}})$ |
|---------|--------------------------|------------------------|-------------------------------|
| 0 | 39.9 | | |
| 300 | 29.1 | | |
| 600 | 21.3 | | |
| 900 | 15.5 | | |
| 1200 | 10.6 | | |
| 1500 | 6.2 | | |
| 1800 | 2.4 | | |
| 2100 | −0.3 | | |
| 2400 | −2.5 | | |
| 2700 | −4.5 | | |
$\alpha_{\text{final}} = -12.0$
[2]
(b) (i) Plot a graph on the grid on page 9 to show how $\log_{10}(\alpha - \alpha_{\text{final}})$ varies with time, $t$.
Use a cross (×) to plot each data point. Draw the line of best fit. [2]
(ii) State and explain whether the results and your graph confirm the relationship $\log_{10}(\alpha - \alpha_{\text{final}}) = A − \frac{kt}{2.30}$. [1]
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(c) (i) Determine the gradient of the graph.
State the co-ordinates of both points you used for your calculation.
Record the value of the gradient to three significant figures.
co-ordinates 1 ............................................ co-ordinates 2 ............................................
gradient = ................................................ $\text{s}^{-1}$ [2]
(ii) Use the gradient value to calculate a value for $k$ in the expression shown.
$\log_{10}(\alpha - \alpha_{\text{final}}) = A − \frac{kt}{2.30}$. [2]
(d) The graph below shows the results obtained from a second hydrolysis of sucrose reaction performed at a different temperature.
(i) The point at time = 2000s is considered to be anomalous.
Suggest what caused the anomaly. [1]
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(ii) Use the graph to determine the half-life, $t_{\frac{1}{2}}$, of this reaction.
State the co-ordinates of both points you used in your calculation.
co-ordinates 1 ............................................ co-ordinates 2 ............................................
half-life = ................................................ $\text{s}$ [2]
(iii) For a first-order reaction, the following relationship exists.
half-life, $t_{\frac{1}{2}} = \frac{0.693}{k'}$.
Use this relationship and your answer to (ii) to determine $k'$, the rate constant for this second hydrolysis reaction.
If you have been unable to determine the half-life of the reaction in (ii), you may use the value $t_{\frac{1}{2}} = 500 \text{s}$, though this is not the correct answer.
$k' = ................................................$ $\text{s}^{-1}$ [1]
(iv) State whether the temperature of the second reaction was higher or lower than that of the first.
Explain your answer with reference to the answers you obtained in (c)(ii) and (d)(iii).
If you have been unable to calculate a value for $k$ in (c)(ii), you may use the value $k = 8.00 \times 10^{-4}$, though this is not the correct answer. [1]
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(v) Would the value of the half-life change if the reaction were repeated with twice the initial concentration of sucrose? Give a reason for your answer. [1]
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584
557
583
