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Verdigris is a green pigment that contains both copper(II) carbonate, CuCO$_3$, and copper(II) hydroxide, Cu(OH)$_2$, in varying amounts.
Both copper compounds react with dilute hydrochloric acid.
$$ \text{CuCO}_3 (s) + 2\text{HCl} (aq) \rightarrow \text{CuCl}_2(aq) + \text{CO}_2(g) + \text{H}_2\text{O} (l) $$
$$ \text{Cu(OH)}_2(s) + 2\text{HCl}(aq) \rightarrow \text{CuCl}_2(aq) + 2\text{H}_2\text{O}(l) $$
(a) You are to plan an experiment to determine the percentage of copper(II) carbonate in a sample of verdigris. Your method should involve the reaction of verdigris with excess dilute hydrochloric acid.
You are provided with the following materials.
• 0.5g of verdigris
• 10.0mol dm$^{-3}$ hydrochloric acid, HCl(aq)
• commonly available laboratory reagents and equipment
You may assume that any other material present in verdigris is unaffected by heating and is not acidic or basic.
(i) Explain why a titration would not be a suitable method to determine the percentage of copper(II) carbonate in a sample of verdigris.
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(ii) The 10.0mol dm$^{-3}$ HCl(aq) is corrosive and too concentrated for use in the experiment.
Describe how you would accurately prepare 250cm$^3$ of 0.500mol dm$^{-3}$ hydrochloric acid from the 10.0mol dm$^{-3}$ HCl(aq) provided.
Include details of any apparatus, including their capacities in cm$^3$, you would use.
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(iii) Identify a dependent variable that you could measure to determine the percentage of copper(II) carbonate in verdigris.
Your answer should be based on a difference that you can identify between the reactions of copper(II) carbonate and copper(II) hydroxide with excess dilute hydrochloric acid.
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(iv) Draw a diagram to show how you would set up apparatus and chemicals to measure the dependent variable identified in (iii).
Label your diagram.
[2]
(v) Using the axes below, sketch a graph to show how the dependent variable you identified in (iii) would change during your experiment.
Extend the graph beyond the point at which the reaction is complete.
Label both axes.
[2]
(vi) A student carries out this experiment once.
Describe how this one experiment should be carried out to ensure that the results are as accurate as possible.
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(vii) A student suspected that their 0.5g sample of verdigris only contained CuCO$_3$.
Calculate the minimum volume, in cm$^3$, of 0.500mol dm$^{-3}$ HCl(aq) that would be needed for the complete reaction of the sample if the student was correct.
$\text{[M}_r: \text{CuCO}_3 = 123.5]$
volume of 0.500mol dm$^{-3}$ HCl(aq) = .......................... cm$^3$ [2]
(b) The following information gives some of the hazards associated with the chemicals used in the procedure.
[Table_1]
Describe one relevant precaution, other than eye protection and a lab coat, that should be taken to keep the risk associated with the chemicals used to a minimum. Explain your answer.
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(c) Azurite is a blue copper-containing mineral. The copper compound in azurite has the formula Cu$_3$(CO$_3$)$_2$(OH)$_2$. This copper compound reacts with sulfuric acid according to the reaction shown.
$$ \text{Cu}_3(\text{CO}_3)_2(\text{OH})_2(s) + 3\text{H}_2\text{SO}_4(aq) \rightarrow 3\text{CuSO}_4(aq) + 2\text{CO}_2(g) + 4\text{H}_2\text{O}(l) $$
A student performed a series of titrations on 1.50g samples of solid azurite using 0.400mol dm$^{-3}$ sulfuric acid.
It can be assumed that any other material present in azurite does not react with sulfuric acid.
The titration data is given in the table.
[Table_2]
The indicator for the titration was bromophenol blue.
The student concluded that 24.15cm$^3$ of 0.400mol dm$^{-3}$ sulfuric acid completely neutralised 1.50g of azurite.
(i) Using the student's value of 24.15cm$^3$, calculate the percentage by mass of Cu$_3$(CO$_3$)$_2$(OH)$_2$ in the sample of azurite.
Write your answer to three significant figures.
$\text{[M}_r: \text{Cu}_3(\text{CO}_3)_2(\text{OH})_2 = 344.5]$
percentage by mass of Cu$_3$(CO$_3$)$_2$(OH)$_2$ in the sample of azurite = .......................... % [3]
(ii) Identify two possible problems with the student's titration and suggest improvements to it.
Problem 1 ..............................................................
Improvement 1 ..............................................................
Problem 2 ..............................................................
Improvement 2 .............................................................. [3]
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Activated charcoal is a form of carbon with a very high surface area. It can be used to remove impurities from mixtures. It does this by a process called adsorption, where particles of the impurity bond (adsorb) to the activated charcoal surface.
A student wanted to determine the ability of activated charcoal to adsorb a blue dye (the impurity) from aqueous solution.
The equation that links the mass of activated charcoal with the amount of blue dye adsorbed is shown.
$$ \log\left(\frac{D}{m}\right) = A + b\log[X] $$
\(D =\) difference in concentration of dye (in g dm\(^{-3}\)) before and after adsorption
\(m =\) mass of activated charcoal (in g)
\([X] =\) final concentration of dye (in g dm\(^{-3}\)) after adsorption
\(A\) and \(b\) are constants
The student used the following procedure to investigate the ability of activated charcoal to adsorb a blue dye from an aqueous solution.
- Place a 50.0 cm\(^3\) sample of a 25.00 g dm\(^{-3}\) solution of blue dye in a conical flask.
- Add a weighed mass of activated charcoal to the flask.
- Stir the contents of the flask for three minutes and then leave for one hour.
- Filter the mixture.
- Determine the final concentration of the blue dye, \([X]\), by colorimetry.
- Repeat the procedure using different masses of activated charcoal.
(a) The final concentrations of blue dye after carrying out the procedure, \([X]\), are shown in the table.
(i) Process the results to complete the table.
Record your data to two decimal places.
[Table_1]
(ii) By considering the data in the first three columns, state the effect of increasing the mass of activated charcoal, \(m\), on the amount of adsorption that occurs.
Explain this effect.
(a)(i) [3]
(a)(ii) [2]
(b) Plot a graph on the grid to show the relationship between \(\log\left(\frac{D}{m}\right)\) and \(\log[X]\).
Use a cross (\(\times\)) to plot each data point. Draw the straight line of best fit.
(b) [2]
(c) Circle the most anomalous point on the graph.
Suggest a reason why this anomaly may have occurred during the experimental procedure.
(c) [2]
(d) (i) Use the graph to determine the gradient of the best-fit line. State the co-ordinates of both points you used in your calculation.
Determine the value of the constant \(b\).
co-ordinates 1 ......................................... co-ordinates 2 .........................................
gradient = ................................................
\(b =\) .....................................................
(d) (i) [2]
(ii) Use the graph to determine a value for \(A\).
\(A =\) .....................................................
(d) (ii) [1]
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