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The concentration of aqueous ammonia used in qualitative analysis is $2\,\text{mol}\,\text{dm}^{-3}$ but it is supplied in a much more concentrated form. This is referred to as '.880 ammonia'. You are to determine the concentration of '.880 ammonia' by titration of a solution of ammonia, FB 1, with hydrochloric acid of known concentration. The equation for the reaction is given below.
$$\text{NH}_3(\text{aq}) + \text{HCl}(\text{aq}) \rightarrow \text{NH}_4\text{Cl}(\text{aq})$$
FB 1 is a dilute solution of ammonia, $\text{NH}_3(\text{aq})$. It was prepared by measuring out $5.91\,\text{cm}^3$ of the '.880 ammonia' and then adding distilled water until the solution had a volume of $1\,\text{dm}^3$.
FB 2 is $0.100\,\text{mol}\,\text{dm}^{-3}$ hydrochloric acid, $\text{HCl}(\text{aq})$.
methyl orange indicator
(a) Method
● Fill the burette with FB 2.
● Use the pipette to transfer $25.0\,\text{cm}^3$ of FB 1 into a conical flask.
● Add a few drops of methyl orange indicator.
● Perform a rough titration and record your burette readings in the space below.
The rough titre is ...................... $\text{cm}^3$.
● Carry out as many accurate titrations as you think necessary to achieve consistent results.
● Make certain any recorded results show the precision of your practical work.
● Record, in a suitable form below, all of your burette readings and the volume of FB 2 added in each accurate titration.
[Table of readings]
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.
25.0 $\text{cm}^3$ of FB 1 required ............ $\text{cm}^3$ of FB 2. [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 hydrochloric acid present in the volume of FB 2 calculated in (b).
moles of $\text{HCl} = \text{......................}$ mol
(ii) Use your answer to (i) to determine the number of moles of ammonia present in $25.0\,\text{cm}^3$ of FB 1, pipetted into the conical flask.
moles of $\text{NH}_3 = \text{......................}$ mol
(iii) Use your answer to (ii) to calculate the concentration, in $\text{mol}\,\text{dm}^{-3}$, of the diluted ammonia, FB 1.
concentration of $\text{NH}_3$ (diluted) in FB 1 = \text{......................}$\text{mol}\,\text{dm}^{-3}$
(iv) Use your answer to (iii) and the information on page 2 to calculate the concentration, in $\text{mol}\,\text{dm}^{-3}$, of '.880 ammonia'.
concentration of '.880 ammonia' = \text{......................}$\text{mol}\,\text{dm}^{-3}$ [3]
(d) A student analysed a different sample of concentrated ammonia and determined the concentration to be $15.0\,\text{mol}\,\text{dm}^{-3}$. Calculate the percentage difference in concentration of the '.880 ammonia' you have determined compared with that of the student.
If you have been unable to complete the calculation, assume the concentration of '.880 ammonia' was $9.35\,\text{mol}\,\text{dm}^{-3}$. This is not the correct value.)
percentage difference in concentration = \text{......................} % [1]
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You are to determine the enthalpy change of solution, in kJ mol^{-1}, of ammonium chloride.
FB 3 is approximately 9 g of solid ammonium chloride, $\text{NH}_4\text{Cl}$.
distilled water
(a) Method
Read through the instructions carefully and prepare a table below for your results before starting any practical work.
- Weigh the plastic cup and record the balance reading.
- Add between 2.9 and 3.1 g of ammonium chloride, FB 3, and record the new balance reading.
- Place the plastic cup in the 250 cm$^3$ beaker.
- Pour 25 cm$^3$ of distilled water into the measuring cylinder.
- Place the thermometer in the water and record the initial temperature in the table of results.
- Pour the 25 cm$^3$ of distilled water into the plastic cup.
- Stir the contents of the cup and record the lowest temperature of the solution. Tilt the cup if necessary to ensure the thermometer bulb is fully immersed.
- Repeat the procedure using the other plastic cup. Use between 4.9 and 5.1 g of FB 3 for this experiment.
- Record the mass of FB 3 used and the change in temperature for each experiment.
Results
(b) (i) Calculate the mean mass of FB 3 used. Give your answer to two decimal places.
mean mass of FB 3 =
(ii) Calculate the mean temperature change. Give your answer to one decimal place.
mean temperature change =
(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 $\text{NH}_4\text{Cl}$ present in the mass of FB 3 calculated in (b)(i).
[A$_r$: H, 1.0; N, 14.0; Cl, 35.5]
moles of $\text{NH}_4\text{Cl}$ =
(ii) Use your answers to (b)(ii) and (c)(i) to calculate the enthalpy change of solution, in kJ mol^{-1}, of $\text{NH}_4\text{Cl}$.
(Assume that 4.2 J of heat energy changes the temperature of 1.0 cm$^3$ of solution by 1.0°C.)
enthalpy change = ......
(d) (i) Which of the two experiments in (a), the first or the second, had the greater percentage error in the value calculated for the temperature change?
(ii) Given that the error in a single thermometer reading is ±0.5°C, calculate the percentage error in the temperature change for the experiment you gave in (d)(i).
percentage error =
(e) (i) A student suggested that it would have been better to calculate the enthalpy change for each experiment separately. What would be the advantage of this suggestion? Explain your answer fully.
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(a) FB 4 and FB 5 are solutions of salts each containing one cation and one anion from those listed in the Qualitative Analysis Notes on pages 10 and 11. Carry out the following tests and record your observations in the table below.
| test | observations |
|---|---|
| FB 4 | FB 5 |
(i) To a 1 cm depth of solution in a test-tube, add aqueous ammonia.
(ii) To a 1 cm depth of solution in a test-tube, add a few drops of aqueous silver nitrate.
(iii) To a 1 cm depth of solution in a test-tube add a few drops of aqueous barium chloride or barium nitrate.
(iv) Identify both ions in FB 4.
cation .............................................................. anion ..............................................................
(v) Suggest the ions which may be present in FB 5.
cations .............................................................. anions ..............................................................
(vi) Select a reagent which could be used in a further test on FB 5 to identify the cation present. Carry out your test and record your observations.
| test | observations |
|---|
To a 1 cm depth of FB 5 in a test-tube, add ..............................................................
The cation in FB 5 is ..............................................................
(b) FB 6 is a pale purple salt containing two cations.
(i) What does this suggest about the identity of one of the cations in FB 6?
..................................................................................................................................................
Carry out the following tests and complete the table below.
| test | observations |
|---|
(ii) Place a spatula measure of FB 6 in a hard-glass test-tube. Heat gently.
(iii) Dissolve a small spatula measure of FB 6 in a 2 cm depth of distilled water in a test-tube. Use this solution for tests (iv) and (v).
(iv) Pour about half the solution prepared in (iii) into a boiling tube and add aqueous sodium hydroxide, then .............................................................. gently warm the mixture.
(v) To the remainder of the solution prepared in (iii), add a few drops of aqueous potassium iodide, then .............................................................. add a few drops of starch solution.
(vi) Identify the cations present in FB 6.
FB 6 contains .............................................................. and ..............................................................
(vii) What type of reaction occurred when potassium iodide was added to FB 6 in (v)?
..................................................................................................................................................
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