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In this experiment you will determine x in the formula of hydrated sodium carbonate, Na$_2$CO$_3$.xH$_2$O, by titration.
FA 1 is hydrated sodium carbonate, Na$_2$CO$_3$.xH$_2$O.
FA 2 is 0.110 mol dm$^{-3}$ hydrochloric acid, HCl.
methyl orange indicator
(a) Method
Making a solution of FA 1
• Record all masses in the space below.
• Weigh the container with FA 1.
• Tip all the solid FA 1 into the 250 cm$^3$ beaker.
• Weigh the container with any residual FA 1.
• Add approximately 100 cm$^3$ of distilled water to the beaker and stir to dissolve FA 1.
• Transfer the solution to the 250 cm$^3$ volumetric flask.
• Rinse the beaker twice, each time with about 20 cm$^3$ of distilled water, and add this to the volumetric flask.
• Add distilled water to the volumetric flask to make 250 cm$^3$ of solution and shake thoroughly. Label this solution FA 3.
• Calculate and record the mass of FA 1 used to make this solution.
Titration
• Pipette 25.0 cm$^3$ of FA 3 into a conical flask.
• Fill the burette with FA 2.
• Add several drops of methyl orange indicator to the conical flask.
• Carry out a rough titration and record your burette readings in the space below.
The rough titre is ..................... cm$^3$.
(b) From your accurate titration results, obtain a suitable value for the volume of FA 2 to be used in your calculations. Show clearly how you obtained this value.
25.0 cm$^3$ of FA 3 required ..................... cm$^3$ of FA 2 [1]
(c) Calculations
(i) Give your answers to (ii), (iii) and (iv) to the appropriate number of significant figures. [1]
(ii) Calculate the number of moles of hydrochloric acid in the volume of FA 2 calculated in (b).
moles of HCl = ............................... mol [1]
(iii) Complete the equation below and include the missing state symbols.
Na$_2$CO$_3$(aq) + HCl(aq) → NaCl + CO$_2$ + H$_2$O
Calculate the number of moles of sodium carbonate in 25.0 cm$^3$ of FA 3.
moles of Na$_2$CO$_3$ in 25.0 cm$^3$ of FA 3 = ............................... mol [1]
(iv) Calculate the number of moles of sodium carbonate in 250.0 cm$^3$ of FA 3.
moles of Na$_2$CO$_3$ in 250.0 cm$^3$ of FA 3 = ............................... mol
Use this answer and your data on page 2 to calculate the relative formula mass, M$_r$, of hydrated sodium carbonate, Na$_2$CO$_3$.xH$_2$O.
M$_r$ of Na$_2$CO$_3$.xH$_2$O = ......................... [1]
(v) Calculate the value of x in Na$_2$CO$_3$.xH$_2$O. Give your answer to the nearest whole number.
x = ............................... [1]
(d) A student suggested using 0.110 mol dm$^{-3}$ sulfuric acid in place of the 0.110 mol dm$^{-3}$ hydrochloric acid used in the experiment above. The mass of FA 1 used was unchanged.
Explain what effect this change would have on the accuracy of the experiment.
...............................
...............................
...............................
...............................
[2]
565
584
550
In Question 1 you used a titration method to investigate a hydrated compound. In Question 2 you will use a method involving measuring masses. You will find the identity of a Group 2 element, Y, whose hydrated sulfate has the formula $YSO_4.7H_2O$.
When heated, the hydrated sulfate loses its water of crystallisation to form anhydrous sulfate. The anhydrous sulfate does not decompose at the temperature of the Bunsen flame.
$$YSO_4.7H_2O(s) \rightarrow YSO_4(s) + 7H_2O(g)$$
FA 4 is the hydrated sulfate of Y, $YSO_4.7H_2O$.
(a) Method
- Weigh the crucible with its lid and record the mass.
- Tip between 1.80 g and 2.00 g of FA 4 into the crucible. Keep the remaining FA 4 for Question 3.
- Weigh and record the mass of crucible, lid and FA 4.
- Place the crucible on the pipe-clay triangle on the tripod. Put the lid on the crucible and heat gently for about 1 minute.
- Use tongs to remove the lid and heat the crucible strongly for about 4 minutes. Replace the lid and then leave to cool.
- While the crucible is cooling, begin work on Question 3.
- When cool, reweigh the crucible with its lid and contents and record the mass.
- Calculate and record the mass of FA 4 before heating, the mass of residue after heating and the mass of water lost.
(b) Calculations
(i) Calculate the number of moles of water lost on heating FA 4.
\[ \text{moles of } H_2O \text{ lost } = \text{.........................} \text{ mol} \] [1]
(ii) Deduce the number of moles of anhydrous $YSO_4$ that are formed when this water is lost.
\[ \text{moles of } YSO_4 = \text{.........................} \text{ mol} \] [1]
(iii) Use your answer to (ii) and the mass of residue left after heating FA 4 to determine the relative atomic mass, $A_r$, of Y.
\[ A_r \text{ of } Y = \text{..........................} \] [2]
(iv) Identify Y.
Y is .......................................................... [1]
(c) A student did not heat the sample of FA 4 for long enough to remove all the water.
What would be the effect of this on the calculated value of the relative atomic mass of Y?
Explain your answer.
[2]
565
507
546
(a) In Question 2 you used a gravimetric method to identify the cation, Y, present in FA 4. You will now use a qualitative analysis method to confirm whether your identification of Y was correct.
Transfer a spatula measure of FA 4 into a boiling tube. Add a 5 cm depth of distilled water and shake the tube to dissolve the solid.
(i) Use 1 cm depths of this solution in test-tubes to carry out tests to identify the cation, Y, present in FA 4. Record your tests and observations in a suitable form in the space below.
(ii) Do your qualitative analysis tests in (i) confirm your identity of Y in Question 2? Explain your answer.
(b) FA 5 contains two of the ions listed in the Qualitative Analysis Notes.
(i) Place a small spatula measure of FA 5 into a hard-glass test-tube and heat, gently at first and then strongly. Record your observations.
(ii) Transfer the remaining FA 5 into the 100 cm³ beaker. Add approximately 20 cm³ of distilled water and stir to form a solution.
For each of the tests below use a separate 1 cm depth of this solution in a test-tube. Record your observations.
| test | observations |
| --- | --- |
| Add aqueous sodium hydroxide. | |
| Add a few drops of acidified potassium manganate(VII), then | |
| add a few drops of ammonium thiocyanate. Tip the contents of the tube down the sink and rinse the tube and sink with tap water. | |
| Add a 1 cm depth of hydrogen peroxide, then | |
| add aqueous sodium hydroxide. | |
| Add a 1 cm depth of dilute nitric acid and then a few drops of aqueous silver nitrate. | |
| Add a few drops of aqueous barium nitrate or aqueous barium chloride, then | |
| add dilute hydrochloric acid. | |
(iii) Identify the ions present in FA 5.
ions present ................................................ and ................................................ [1]
(iv) What type of reaction is occurring when FA 5 reacts with acidified potassium manganate(VII)?
500
587
593
