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Limewater is a saturated solution of calcium hydroxide. A sample of limewater was diluted to produce solution FA 1. You are to determine the concentration of calcium hydroxide, $Ca(OH)_2$, in FA 1 by titration with hydrochloric acid, $HCl$, which you will first dilute. The equation for the reaction between calcium hydroxide and hydrochloric acid is given below.
$$Ca(OH)_2(aq) + 2HCl(aq) \rightarrow CaCl_2(aq) + 2H_2O(l)$$
FA 1 is aqueous calcium hydroxide, $Ca(OH)_2$.
FA 2 is 0.500 mol $dm^{-3}$ hydrochloric acid, $HCl$.
methyl orange indicator
(a) Method
Read through the method before starting any practical work.
(i) Dilution
\[ \begin{align*}
&\bullet \text{ Fill the burette with FA 2.} \
&\bullet \text{ Run between 24.00 and 26.00 cm}^3 \text{ of FA 2 into the 250 cm}^3 \text{ volumetric flask.} \
&\bullet \text{ Record your burette readings and the volume used in the space below.} \
&\bullet \text{ Make up the solution to 250 cm}^3 \text{ with distilled water and shake the flask to ensure thorough mixing. This solution is FA 3.}
\end{align*} \]
\[\text{volume of 0.500 mol}dm^{-3} \hspace{.05in} HCl \hspace{.05in} \text{used = ................................... cm}^3 \]
(ii) Titration
\[ \begin{align*}
&\bullet \text{ Empty and rinse the burette.} \
&\bullet \text{ Fill the burette with FA 3.} \
&\bullet \text{ Pipette 25.0 cm}^3 \text{ of FA 1 into a conical flask.} \
&\bullet \text{ Add a few drops of methyl orange indicator.} \
&\bullet \text{ Perform a rough titration and record your burette readings in the space below.}
\end{align*} \]
\[\text{The rough titre is .............................. cm}^3. \]
\[ \begin{align*}
&\bullet \text{ Carry out as many accurate titrations as you think necessary to obtain consistent results.} \
&\bullet \text{ Make certain any recorded results show the precision of your practical work.} \
&\bullet \text{ Record, in a suitable form below, all of your burette readings and the volume of FA 3 added in each accurate titration.}
\end{align*} \]
\[ \begin{array}{|c|}
\hline
I \\\hline
II \\\hline
III \\\hline
IV \\\hline
V \\\hline
VI \\\hline
VII \\\hline
VIII \\\hline
\end{array} \]
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
\[ 25.0 cm^3 \text{ of FA 1 required ..................... cm}^3 \text{ of FA 3.} \]
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use your volume of FA 2 from (a)(i) to calculate the concentration of FA 3.
\[ \text{concentration of FA 3 = ........................ mol dm}^{-3} \]
(ii) Use your answer to (c)(i) and the value calculated in (b) to calculate the number of moles of hydrochloric acid used to neutralise 25.0 cm$^3$ of FA 1.
\[ \text{moles of HCl = ........................ mol} \]
(iii) Use your answer to (c)(ii) to calculate the number of moles of calcium hydroxide in 25.0 cm$^3$ of FA 1.
\[ \text{moles of Ca(OH)}_2 \text{ = ........................ mol} \]
(iv) Use your answer to (c)(iii) to calculate the concentration, in mol $dm^{-3}$, of calcium hydroxide in FA 1.
\[ \begin{array}{|c|}
\hline
I \\\hline
II \\\hline
III \\\hline
IV \\\hline
V \\\hline
\end{array} \]
\[ \text{concentration of Ca(OH)}_2 \text{ = ........................ mol }dm^{-3} \tag{5} \]
(d) (i) The maximum error in a single burette reading is ±0.05 cm$^3$. When making up the diluted acid, FA 3, a student recorded that 24.80 cm$^3$ of FA 2 was used. What are the smallest and largest possible volumes of acid that were run into the volumetric flask?
\[ \text{smallest volume used = ............... cm}^3 \text{ largest volume used = ............... cm}^3 \]
(ii) If the actual volume of acid added had been less than 24.80 cm$^3$, how would the value the student calculated for the concentration of $Ca(OH)_2$ in (c)(iv) compare to the true value? Explain your answer.
\[ \text{..........................................................................................................................................................................................} \]
\[ \text{..........................................................................................................................................................................................} \]
\[ \text{..........................................................................................................................................................................................} \]
[2]
(e) If a sample of limewater is left open to the air, the concentration of calcium hydroxide decreases. Explain why.
\[ \text{..........................................................................................................................................................................................} \]
\[ \text{..........................................................................................................................................................................................} \]
[1]
556
586
504
You are to determine the percentage by mass of zinc carbonate in a sample of powdered zinc carbonate ore by means of thermal decomposition. You may assume that none of the other components of the ore is affected by heating. The equation for the reaction occurring is given below.
$\text{ZnCO}_3(s) \rightarrow \text{ZnO}(s) + \text{CO}_2(g)$
FA 4 is zinc carbonate ore.
(a) Method
Read through the method before starting any practical work and prepare a table for your results in the space below.
● Weigh the empty crucible and record the mass in your table.
● Transfer all the FA 4 into the crucible.
● Weigh the crucible with FA 4 and record the mass.
● Place the crucible on the pipe-clay triangle.
● Heat the crucible gently for about one minute and then strongly for four minutes.
● Remove the Bunsen burner and allow the crucible to cool.
● While the crucible is cooling start working on another question.
● Reweigh the cooled crucible with contents and record the mass.
● Record the mass of FA 4 used and the mass of solid remaining after heating.
● Beneath your table, record any observations you have made while the solid was heated and cooled.
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) From your results in (a), calculate the mass of carbon dioxide lost on heating FA 4.
mass of $\text{CO}_2$ lost = ................. g
(ii) Use your answer to (i) to calculate the mass of zinc carbonate present in the sample of FA 4 that was heated.
[Ar: C, 12.0; O, 16.0; Zn, 65.4]
mass of $\text{ZnCO}_3$ = ................. g
(iii) Calculate the percentage by mass of zinc carbonate in the zinc carbonate ore.
percentage of $\text{ZnCO}_3$ = ................. % [4]
(c) Suggest how a student, using all the same apparatus, could alter the method to be more confident that the percentage of zinc carbonate is correct. Explain your answer.
[2]
535
542
510
(a) You are provided with a different sample of zinc carbonate ore, labelled FA 5. This sample contains zinc carbonate, ZnCO₃, and one other salt. This additional salt contains a single cation and a single anion from those listed on pages 11 and 12. By carrying out the following tests you will be able to suggest the identities of the ions in the additional salt.
[Table_1: test observations]
(i) Transfer the solid FA 5 into a boiling tube and add a 3 cm depth of dilute nitric acid to make solution FA 6.
Use this solution in tests (ii) to (v).
(ii) To a 1 cm depth of FA 6 in a test-tube, add aqueous sodium hydroxide.
(iii) To a 1 cm depth of FA 6 in a test-tube, add aqueous ammonia.
(iv) To a 1 cm depth of FA 6 in a test-tube, add a 1 cm depth of aqueous silver nitrate.
(v) To a 1 cm depth of FA 6 in a test-tube, add a 1 cm depth of aqueous barium chloride or barium nitrate.
(vi) Suggest the identity of the cation and anion (apart from Zn²⁺ and CO₃²⁻) present in FA 5. Explain your choice.
cation ....................
reason ..................................................................................................................
anion ....................
reason ..................................................................................................................
(b) You are provided with solid FA 7, which is a mixture of two salts. Use separate portions of FA 7 to perform the experiments below.
[Table_2: test observations]
(i) Heat a spatula measure of FA 7 in a hard-glass test-tube gently at first, then
..................................................................................................................
heat more strongly and test the gas evolved with litmus paper.
Dissolve a spatula measure of FA 7 in a test-tube with a 5 cm depth of distilled water for use in tests (ii) and (iii).
(ii) To a 1 cm depth of FA 7(aq) in a test-tube, add aqueous silver nitrate,
..................................................................................................................
followed by aqueous ammonia.
(iii) To a 1 cm depth of FA 7(aq) add barium chloride or barium nitrate,
..................................................................................................................
followed by dilute nitric acid.
(iv) Use your observations and the Qualitative Analysis Notes from pages 11 and 12 to identify three of the ions present. Give evidence for your choice of ions.
ion .................... evidence ..................................................................................
..................................................................................................................
ion .................... evidence ..................................................................................
..................................................................................................................
ion .................... evidence ..................................................................................
..................................................................................................................
533
506
531
