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You are to determine the percentage by mass of water in the borax crystals.
Borax reacts with hydrochloric acid according to the equation.
$\text{Na}_2\text{B}_4\text{O}_7(aq) + 2\text{HCl}(aq) + 5\text{H}_2\text{O}(l) \rightarrow 2\text{NaCl}(aq) + 4\text{H}_3\text{BO}_3(aq)$
FA 1 is an aqueous solution containing 38.10 g dm$^{-3}$ of borax crystals. Borax has the formula, $\text{Na}_2\text{B}_4\text{O}_7.x\text{H}_2\text{O}$.
FA 2 is 1.00 mol dm$^{-3}$ hydrochloric acid, HCl.
You are also provided with an indicator suitable for the titration of a strong acid and a weak base.
The indicator provided is ...................................................
(a) Method
Dilution
- Fill the burette with FA 2.
- Run between 44.50 cm$^3$ and 45.50 cm$^3$ of FA 2 from the burette into the 250 cm$^3$ graduated (volumetric) flask, labelled FA 3.
- Make the solution up to the mark with distilled water.
- Shake the flask to mix the solution of FA 3.
Titration
- Fill a second burette with FA 3, the diluted hydrochloric acid.
- Pipette 25.0 cm$^3$ of FA 1 into a conical flask.
- Add to the flask a few drops of the indicator provided.
- Titrate the borax in the flask with FA 3 until the appropriate colour change is observed for the end-point.
In the space below record your burette readings for this rough titration.
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 in a suitable form below all of your burette readings and the volume of FA 3 added in each accurate titration.
(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 1 required ............. cm$^3$ of FA 3.
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate the concentration of hydrochloric acid, in mol dm$^{-3}$, in the diluted solution, FA 3.
The concentration of HCl in FA 3 was ....................... mol dm$^{-3}$.
(ii) Calculate how many moles of HCl were present in the volume of FA 3 calculated in (b).
.................. mol of HCl
(iii) Calculate how many moles of $\text{Na}_2\text{B}_4\text{O}_7$ reacted with the HCl in (ii).
$\text{Na}_2\text{B}_4\text{O}_7(aq) + 2\text{HCl}(aq) + 5\text{H}_2\text{O}(l) \rightarrow 2\text{NaCl}(aq) + 4\text{H}_3\text{BO}_3(aq)$
The HCl run from the burette reacted with ................... mol of $\text{Na}_2\text{B}_4\text{O}_7$.
(iv) Calculate the concentration, in g dm$^{-3}$, of $\text{Na}_2\text{B}_4\text{O}_7$ in FA 1.
[A$_r$: B, 10.8; O, 16.0; Na, 23.0]
The concentration of $\text{Na}_2\text{B}_4\text{O}_7$ in FA 1 is .................. g dm$^{-3}$.
(v) Use your answer to (iv) and the information at the start of question 1 to calculate the percentage by mass of water in the borax crystals, $\text{Na}_2\text{B}_4\text{O}_7.x\text{H}_2\text{O}$.
[A$_r$: H, 1.0; B, 10.8; O, 16.0; Na, 23.0]
Borax crystals contain ................. % water.
(d) The maximum error for a 25 cm$^3$ pipette commonly used in schools is ±0.06 cm$^3$.
The maximum error in any single burette reading is ±0.05 cm$^3$.
Calculate the maximum percentage error in each of the following.
(i) The volume of FA 1 pipetted into the conical flask.
maximum percentage error in the pipetted volume = ....................... %.
(ii) The titre volume calculated in (b).
maximum percentage error in titre volume = ....................... %.
517
551
591
Method
(a) sodium carbonate
- Support the plastic cup in a 250 cm³ beaker.
- Use a measuring cylinder to transfer 50 cm³ of FA 7 into the plastic cup.
- Measure and record the temperature of the acid in the cup.
- Measure and record the mass of the container labelled FA 4, containing Na₂CO₃.
- Carefully tip the sodium carbonate from the weighed container FA 4 into the hydrochloric acid in the plastic cup.
Note: There will be vigorous effervescence. Do not breathe the vapour. Add the solid in small portions with constant stirring using the thermometer. - Record the highest temperature obtained.
- Reweigh the container FA 4 with any residual sodium carbonate. Record the mass.
- Empty and rinse the plastic cup and dry it using a paper towel.
- both balance readings and both temperature measurements,
- the mass of sodium carbonate, $m_1$, used in the experiment,
- the temperature rise, $\Delta T_1$.
$\frac{\Delta T_1}{m_1} = \begin{align}\text{sign} \end{align} + \text{value} \, ^\circ \text{C g}^{-1}$
(b) sodium hydrogencarbonate
- Support the plastic cup in a 250 cm³ beaker.
- Use a measuring cylinder to transfer 50 cm³ of FA 7 into the plastic cup.
- Measure and record the temperature of the acid in the cup.
- Measure and record the mass of the container labelled FA 5, containing NaHCO₃.
- Carefully tip the sodium hydrogencarbonate from the weighed container FA 5 into the hydrochloric acid in the plastic cup.
Note: There will be vigorous effervescence. Add the solid in small portions with constant stirring using the thermometer. - Record the lowest temperature obtained.
- Reweigh the container FA 5 with any residual sodium hydrogencarbonate. Record the mass.
- Empty and rinse the plastic cup and dry it using a paper towel.
- both balance readings and both temperature measurements,
- the mass of sodium hydrogencarbonate, $m_2$, used in the experiment,
- the temperature fall, $\Delta T_2$.
$\frac{\Delta T_2}{m_2} = \begin{align}\text{sign} \end{align} - \text{value} \, ^\circ \text{C g}^{-1}$
(c) mixture of sodium carbonate and sodium hydrogencarbonate
- Support the plastic cup in a 250 cm³ beaker.
- Use a measuring cylinder to transfer 50 cm³ of FA 7 into the plastic cup.
- Measure and record the temperature of the acid in the cup.
- Measure and record the mass of a clean, dry, weighing-bottle or tube.
- Add to the tube between 8.5 g and 9.5 g of the mixture FA 6.
- Record the mass of the weighing-bottle or tube + FA 6.
- Carefully tip the weighed mixture into the hydrochloric acid in the plastic cup.
Note: There will be vigorous effervescence. Add the solid in small portions with constant stirring using the thermometer. - Record the highest or lowest temperature obtained.
- Reweigh the weighing-bottle or tube with any residual mixture. Record the mass.
- all balance readings and temperature measurements,
- the mass of the mixture, $m_3$, used in the experiment,
- the temperature change, $\Delta T_3$.
(d) Transfer the following data from parts (a), (b) and (c).
- (a) $\frac{\Delta T_1}{m_1} = \begin{align}\text{sign} \end{align} + \text{value} \, ^\circ \text{C g}^{-1}$
- (b) $\frac{\Delta T_2}{m_2} = \begin{align}\text{sign} \end{align} - \text{value} \, ^\circ \text{C g}^{-1}$
- (c) $m_3 = \text{......................} \, \text{g} \qquad \Delta T_3 = \begin{align}\text{sign} \end{align} \text{......................} \, ^\circ \text{C}$
mass of sodium carbonate / g = $W$
mass of sodium hydrogencarbonate / g = $(m_3 - W)$
Evaluate the following equation to determine a value for $W$.
\[ [W \times \frac{\Delta T_1}{m_1}] + [(m_3 - W) \times \frac{\Delta T_2}{m_2}] = \Delta T_3\]
The mass of sodium carbonate was ....................... g.
(e) Use the information at the beginning of question 2 to outline an alternative method that could be used in a school laboratory to find the mass of sodium carbonate and the mass of sodium hydrogencarbonate in the mixture FA 6.
..........................................................................................................................................................
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579
501
500
FA 8, FA 9 and FA 10 are aqueous solutions each containing a sodium cation and a single anion which could be a nitrite, a nitrate or a halide.
(a) By reference to the Qualitative Analysis Notes on page 15, select a single reagent that would enable you to identify any solution containing the nitrite ion, $\text{NO}_2^-$.
Reagent: ...........................................................................................................................
Use this reagent to test each of the solutions. Record your observations in the table below. State clearly where no reaction has been observed.
[Table_1]
(b) By reference to the Qualitative Analysis Notes on page 15, select one reagent that would show that a halide ion is present.
Reagent: ...........................................................................................................................
Use this reagent to test each of the solutions. Record your observations in an appropriate form in the space below.
Select another reagent to identify or confirm which halide ions are present in the solutions.
Reagent: ...........................................................................................................................
Tick the appropriate statement about the use of this reagent.
[Table_2]
Use this reagent to identify or confirm which halide ions are present in the solutions and record your observations in an appropriate form in the space below.
(c) From the results in (a) and (b) state which anions have been identified in the solutions.
Complete the following table. Place a cross in any box if no anion has been identified.
[Table_3]
FA 11 and FA 12 are aqueous solutions each containing one cation from those listed in the Qualitative Analysis Notes printed on page 14.
(d) Use aqueous sodium hydroxide and aqueous ammonia in separate tests to identify the cation present in each of the solutions. You will also require some of the solution, FA 11, for tests in (f).
Record the results of your experiments with sodium hydroxide and ammonia in an appropriate form in the space below.
(e) Identification of the cations in FA 11 and FA 12. Complete the table below.
[Table_4]
What is the evidence from your observations in (d) that enables you to identify the cation present in each of the solutions?
The evidence supporting the conclusion for the cation in FA 11 is ...................................................................................................................................................
......................................................................................................................................................
The evidence supporting the conclusion for the cation in FA 12 is ...................................................................................................................................................
......................................................................................................................................................
(f) Complete the following table.
[Table_5]
(i) Pour 1 cm depth of FA 11 into a test-tube.
Add 1 cm depth of aqueous potassium iodide.
Divide this mixture into two parts for use in (ii) and (iii).
(ii) To the first part of the mixture from (i) add a few drops of starch solution.
(iii) To the second part of the mixture from (i) add aqueous sodium thiosulfate, a drop at a time, until no further change is observed.
In part (i) and in part (iii) redox reactions have taken place.
Complete the table below to show the ion or molecule which has been oxidised and the ion or molecule which has been reduced in each of these reactions.
[Table_6]
508
559
520
