No questions found
You are advised to begin work on question 2, and return to question 1 later.
In this experiment you are to investigate the reaction between phosphoric acid and sodium hydroxide in order to determine the chemical equation.
FA 1 is an aqueous solution of phosphoric acid, $H_3PO_4$.
FA 2 is 0.115 moldm$^{-3}$ sodium hydroxide, $NaOH$.
phenolphthalein indicator
(a) Method
**Dilution**
• Weigh the 100cm$^3$ beaker provided. Record the mass in the space below.
• Use the measuring cylinder to add about 10cm$^3$ of FA 1 to the beaker. Weigh the beaker with FA 1 and record the mass.
• Calculate the mass of FA 1 used and record this in the space below.
• Pour the FA 1 from the beaker into the 250cm$^3$ graduated (volumetric) flask provided, labelled FA 3.
• Wash the beaker twice with small quantities of water and add these washings to the volumetric flask.
• Make the solution up to the mark using distilled water. This diluted solution of phosphoric acid is FA 3.
• Ensure that the contents of the flask are thoroughly mixed before using FA 3 for your titrations.
**Titration**
• Fill the burette with FA 2.
• Pipette 25.0cm$^3$ of FA 3 into a conical flask.
• Add 5 drops of phenolphthalein indicator to the flask. The indicator should remain colourless.
• Titrate FA 3 with FA 2 until the indicator changes to a permanent pale pink colour.
• Perform a rough titration and record your burette readings in the space below.
The rough titre is ............ cm$^3$.
• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Make sure 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 2 added in each accurate titration.
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(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
25.0cm$^3$ of FA 3 required ............... cm$^3$ of FA 2.
[1]
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate how many moles of sodium hydroxide were present in the volume of FA 2 calculated in (b).
moles of NaOH = ..................mol
(ii) The phosphoric acid, FA 1, that you weighed out contained 8.40% by mass of $H_3PO_4$. Calculate the mass of $H_3PO_4$ that you weighed out.
mass of $H_3PO_4$ = .................. g
(iii) Calculate how many moles of $H_3PO_4$ were present in 25.0cm$^3$ of the diluted solution, FA 3.
($A_r$: H, 1.0; O, 16.0; P, 31.0)
moles of $H_3PO_4$ = .................. mol
(iv) Use your answers to (i) and (iii) to calculate how many moles of NaOH react with 1 mole of $H_3PO_4$.
Give your answer to the nearest whole number.
moles of NaOH = .......... mol
(v) When NaOH reacts with $H_3PO_4$, the salt formed could be $NaH_2PO_4$, $Na_2HPO_4$ or $Na_3PO_4$.
Use your answer to (iv) to deduce which one of these three salts was the major product formed during the titration.
Write the equation for the reaction of NaOH with $H_3PO_4$ to produce this salt.
[5]
(d) (i) A 25cm$^3$ pipette is accurate to ±0.06cm$^3$.
Calculate the maximum percentage error when the pipette was used to measure solution FA 3.
percentage error in measuring FA 3 = .................. %
(ii) State the maximum error in the mass of the 100cm$^3$ beaker used in the dilution of FA 1.
maximum error = ........... g
(iii) Calculate the maximum percentage error in the mass of FA 1 used.
maximum percentage error = .................. %
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(a) Method
You will carry out the following experiment twice. Record all weighings for the first and second experiments in an appropriate form in the space below.
• Record the mass of an empty crucible without its lid.
• Add 2.6 – 3.4 g of FA 4 into the weighed crucible. Record the mass of the crucible and its contents.
• Use a pipe-clay triangle to support the crucible and contents on a tripod.
• Heat the crucible and its contents gently for about one minute with the lid off. Then heat strongly for a further four minutes.
• Put the lid on the crucible and leave to cool for several minutes.
While you are waiting for the crucible to cool, start work on Question 1.
• When the crucible is cool, remove the lid and weigh the crucible with the anhydrous residue, MX_2.
• Repeat the experiment using the second crucible. This time add 1.6 – 2.4 g of FA 4.
(b) Calculation
Show your working and express your answers to each step to three significant figures.
(i) Calculate the number of moles of water removed from the hydrated salt in the first experiment. $(A_r : H, 1.0; O, 16.0)$
moles of $H_2O = \text{.............. mol}$
(ii) Complete the equation for the removal of water from hydrated FA 4.
Include state symbols.
$$MX_2·2H_2O(.....) \rightarrow ............(......) + ............(......)$$
(iii) Use your equation from (ii) and your answer to (i) to calculate the number of moles of anhydrous $MX_2$ produced in the first experiment.
moles of $MX_2 = \text{........... mol}$
(iv) Use your results from the first experiment to calculate the mass of anhydrous $MX_2$ produced.
mass of $MX_2 = \text{.................. g}$
(v) Calculate the relative formula mass of $MX_2$.
relative formula mass of $MX_2 = \text{..................}$
(c) (i) Suggest how the experimental procedure could be modified to ensure that all of the water of crystallisation had been removed by heating hydrated FA 4.
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(ii) The crucible was cooled with the lid on to prevent absorption of water vapour from the air. Suggest an alternative way of preventing water vapour being absorbed.
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(a) FA 4 contains a halide ion. FA 5 is a solution of a mixture of two salts. It contains one cation and two anions.
- Place a spatula measure of FA 4 into a boiling tube.
- Half fill the boiling tube with distilled water and stir until the solid dissolves. Use this aqueous solution of FA 4 as instructed in the following tests.
(i) Use aqueous silver nitrate and aqueous ammonia to identify the halide ion present in FA 4. Carry out the same test on FA 5. Use a 1 cm depth of solution in a test-tube for each test that you carry out.
Record your observations and conclusions in an appropriate form in the space below.
(ii) Write the ionic equation for the reaction of the halide ion in FA 4 in test (i).
.................................................................................................................................
(iii) Use your conclusion from 3(a)(i) and your answer to 2(b)(v) to calculate the relative atomic mass of metal M in FA 4.
($A_{r}$: F, 19.0; Cl, 35.5; Br, 79.9; I, 127)
(If you were unable to calculate the relative formula mass of anhydrous MX$_2$ in 2(b)(v) assume that it was 222, so that you are able to carry out this calculation).
$A_{r}$ of M = ...........
(iv) The relative atomic masses of some of the cations on page 10 are given below. ($A_{r}$: Mg, 24.3; Ca, 40.0; Fe, 55.8; Cu, 63.5; Mn, 54.9; Zn, 65.4; Ba, 137; Pb, 207)
M is one of the cations listed above. Suggest the identity of M and justify your answer.
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(v) Suggest why it was not necessary to include the cations aluminium and chromium from page 10 in the list of relative atomic masses in (iv).
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(b) (i) Perform the experiments described below and record your observations in the table
Record your observations in the table below.
| test | observation |
|------|-------------|
| To 1 cm depth of aqueous FA 4 in a test-tube, add aqueous ammonia. | |
| To 1 cm depth of FA 5 in a test-tube, add aqueous ammonia. | |
(ii) Use only the results from (i) to identify the two possible cations that would have resulted in your observations for FA 4.
FA 4 contains ................................................... or .....................................................
(iii) Use the results from (i) to identify the cation in FA 5.
FA 5 contains .....................................................
(iv) Use the information on pages 10 and 11 to select one reagent, other than sodium hydroxide, to distinguish between the pair of possible cations in FA 4 identified in (ii).
Carry out the test with your selected reagent, using a 1 cm depth of FA 4.
From this test, identify the cation in FA 4.
reagent ....................................................................................................................
observation ................................................................................................................
FA 4 contains ......................................................................................................
(v) Perform the tests described below and record your observations.
Identify the second anion in FA 5.
| test | observation |
|------|-------------|
| To 1 cm depth of aqueous FA 4 in a test-tube, add 1 cm depth of FA 5. | |
| Then add 2 cm depth of dilute hydrochloric acid. | |
FA 5 contains ......................................................................................................
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