No questions found
Borax is an alkali which has many uses. In this experiment you will determine x in the chemical formula of borax, $Na_2B_xO_7.10H_2O$, by titration with hydrochloric acid.
FB 1 is a solution containing 15.5 g dm$^{-3}$ of borax, $Na_2B_xO_7.10H_2O$.
FB 2 is 2.00 mol dm$^{-3}$ hydrochloric acid, $HCl$.
methyl orange indicator
(a) Method
**Dilution of FB 2**
• Pipette 10.0 cm$^3$ of FB 2 into the 250 cm$^3$ volumetric flask.
• Make the solution up to 250 cm$^3$ using distilled water.
• Shake the solution in the volumetric flask thoroughly.
• This diluted solution of hydrochloric acid is FB 3. Label the volumetric flask FB 3.
**Titration**
• Fill the burette with FB 3.
• Pipette 25.0 cm$^3$ of FB 1 into a conical flask.
• Add several drops of methyl orange.
• 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 FB 3 added in each accurate titration.
(b) From your accurate titration results, obtain a suitable value for the volume of FB 3 to be used in your calculations.
Show clearly how you obtained this value.
25.0 cm$^3$ of FB 1 required ..................... cm$^3$ of FB 3. [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 3 calculated in (b).
moles of $HCl$ = .......................... mol
(ii) 1 mole of borax is neutralised by 2 moles of hydrochloric acid.
Calculate the number of moles of borax that react with the hydrochloric acid in (i).
moles of borax = .......................... mol
(iii) Use your answer to (ii) to calculate the number of moles of borax in 1.00 dm$^3$ of FB 1.
moles of borax in 1.00 dm$^3$ FB 1 = .......................... mol
(iv) Use your answer to (iii) and the information on page 2 to calculate the relative formula mass, $M_r$, of borax.
$M_r$ of borax = ..........................
(v) Calculate x in the formula of borax, $Na_2B_xO_7.10H_2O$.
Use data from the Periodic Table on page 12.
x = ...................... [5]
582
573
542
Some metal carbonates cannot be obtained in a pure state. For example magnesium carbonate exists in a 'basic' form, in which magnesium hydroxide is also present.
One possible chemical formula of basic magnesium carbonate is $\text{MgCO}_3.\text{Mg(OH)}_2.2\text{H}_2\text{O}$.
When basic magnesium carbonate is heated, if the possible formula were correct, it would decompose as shown below.
$$\text{MgCO}_3.\text{Mg(OH)}_2.2\text{H}_2\text{O}(s) \rightarrow 2\text{MgO}(s) + \text{CO}_2(g) + 3\text{H}_2\text{O}(g)$$
In this experiment, you will decompose basic magnesium carbonate by heating it, and you will use your results to determine whether this possible formula is correct.
FB 4 is basic magnesium carbonate.
(a) Method
Read through the method before starting any practical work and prepare a table for your results in the space below.
• Weigh a crucible with its lid and record the mass.
• Add 1.1-1.3 g of FB 4 to the crucible. Weigh the crucible and lid with FB 4 and record the mass.
• Place the crucible on the pipe-clay triangle and remove the lid.
• Heat the crucible and contents gently for about one minute.
• Then heat the crucible and contents strongly for about four minutes.
• Replace the lid and allow the crucible to cool for at least five minutes.
• While the crucible is cooling, you may wish to begin work on Question 3.
• Re-weigh the crucible and contents with lid. Record the mass.
• Calculate, and record, the mass of FB 4 used and the mass of residue obtained.
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use your results to calculate the number of moles of magnesium oxide, MgO, obtained as residue.
moles of MgO obtained = .......................... mol
(ii) Use your answer to (i), with the equation on page 4 and the mass of FB 4 you used, to calculate the relative formula mass, $M_r$, of basic magnesium carbonate.
$M_r$ of basic magnesium carbonate (from experiment) = ...........................
(iii) Use data from the Periodic Table to calculate the relative formula mass, $M_r$, of basic magnesium carbonate from its possible formula, $\text{MgCO}_3.\text{Mg(OH)}_2.2\text{H}_2\text{O}$.
$M_r$ of basic magnesium carbonate (from formula) = ...........................
(iv) If the relative formula mass of basic magnesium carbonate obtained from your experiment is within 2.5% of the answer in (iii), this is good evidence that the possible formula, $\text{MgCO}_3.\text{Mg(OH)}_2.2\text{H}_2\text{O}$, is correct.
Does your experiment support the possible formula? Give a reason for your answer.
.................................................................................................................................
.................................................................................................................................
(c) Evaluation
(i) State one way in which the accuracy of the experimental procedure could have been improved using the same mass of FB 4.
Explain your answer.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
(ii) A student carried out the experiment twice using different masses of FB 4. He used the mean mass of FB 4 and the mean mass of magnesium oxide obtained to calculate the relative formula mass of basic magnesium carbonate.
Instead of doing this, he could have calculated the relative formula mass of basic magnesium carbonate from his two experiments separately.
Suggest one advantage of carrying out separate calculations for each experiment.
.................................................................................................................................
.................................................................................................................................
(iii) State the error when making one reading on your balance.
error = ................. g
Calculate the maximum percentage error in the mass of FB 4 used.
percentage error = ................. %
600
540
576
(a) FB 5, FB 6 and FB 7 are solutions, each of which contain one cation and one anion. The anions present are all listed on page 11.
Use a 1 cm depth of these solutions in a test-tube for each of the following tests.
Complete the table below.
[Table with observations]
Add a 2 cm strip of magnesium ribbon.
Add aqueous sodium hydroxide.
Add an equal depth of aqueous potassium iodide.
Add a few drops of FB 5.
(b) (i) From the observation made when potassium iodide was added to FB 6, suggest the identity of the cation in FB 6. Explain your conclusion.
cation in FB 6 ...........................
explanation ...........................
(ii) FB 5 gives no precipitate when aqueous ammonia is added. Suggest the identities of both ions in FB 5.
cation in FB 5 ...........................
anion in FB 5 ...........................
(iii) Identify FB 7.
...........................
(iv) Give the ionic equation for the reaction between magnesium and FB 7.
...........................
(c) FB 8 is a solid. Carry out the following tests on FB 8.
Record your observations in each test.
(i) Heat a small spatula measure of FB 8 gently in a hard-glass test-tube.
observations ...........................
(ii) To a 1 cm depth of hydrochloric acid in a test-tube, add a small spatula measure of FB 8.
observations ...........................
(iii) What conclusions, if any, can you make about the identities of the ions in FB 8?
cation in FB 8 ...........................
anion in FB 8 ...........................
526
578
594
