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In this experiment you are to determine x in the formula of hydrated sodium carbonate, $$Na_2CO_3·xH_2O$$, by titration.
FA 1 is aqueous sodium carbonate, $$Na_2CO_3$$, containing 150.0 g of solid $$Na_2CO_3·xH_2O$$ in each dm³ of solution.
FA 2 is 0.110 mol dm⁻³ nitric acid, $$HNO_3$$.
methyl orange indicator
(a) Method
**Diluting the solution of FA 1**
⚫ Pipette 25.0 cm³ of FA 1 into the 250 cm³ volumetric (graduated) flask.
⚫ Make the solution up to the mark using distilled water.
⚫ Shake the flask thoroughly to mix the solution before using it for your titrations.
⚫ This solution of sodium carbonate is FA 3.
⚫ Rinse the pipette with distilled water.
**Titration**
⚫ Pipette 25.0 cm³ 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 ............... cm³.
⚫ 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.
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
volume of FA 2 = ............... cm³ [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 nitric acid in the volume of FA 2 calculated in (b).
moles of $$HNO_3$$ = ............... mol
(ii) Complete the equation below and include the missing state symbols.
$$Na_2CO_3(aq) + HNO_3(aq) \rightarrow NaNO_3 + CO_2 + H_2O$$
(iii) Calculate the number of moles of sodium carbonate in 25.0 cm³ of solution FA 3.
moles of $$Na_2CO_3$$ in 25.0 cm³ of FA 3 = ............... mol
(iv) Calculate the number of moles of sodium carbonate in 250.0 cm³ of solution FA 3.
moles of $$Na_2CO_3$$ in 250.0 cm³ of FA 3 = ............... mol
(v) Use the information on page 2 and your answer to (iv) to calculate the relative formula mass, $$M_r$$, of hydrated sodium carbonate, $$Na_2CO_3·xH_2O$$.
$$M_r$$ of $$Na_2CO_3·xH_2O$$ = ..............................
(vi) Calculate the value of x in $$Na_2CO_3·xH_2O$$. Give your answer to the nearest whole number.
[Ar: H, 1.0; C, 12.0; O, 16.0; Na, 23.0]
x = ........................
[6]
(d) (i) State the maximum error in any single reading of the burette.
maximum error = ............... cm³
(ii) Calculate the maximum percentage error in the volume of FA 2 in your first accurate titre.
maximum percentage error = ...............%
[1]
584
529
582
In this experiment you will determine the enthalpy change, $\Delta H$, for the dehydration of hydrated sodium carbonate to anhydrous sodium carbonate.
$$\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}(s) \rightarrow \text{Na}_2\text{CO}_3(s) + x\text{H}_2\text{O}(l)$$
In order to do this, first you will determine the enthalpy changes for the reactions of anhydrous sodium carbonate and hydrated sodium carbonate with excess hydrochloric acid. Then you will use Hess' Law to calculate the enthalpy change for the reaction above.
FA 4 is anhydrous sodium carbonate, $\text{Na}_2\text{CO}_3$.
FA 5 is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$.
FA 6 is 4.0 mol dm$^{-3}$ hydrochloric acid, HCl.
(a) Determination of the enthalpy change for the reaction of anhydrous sodium carbonate, FA 4, with excess hydrochloric acid.
**Method**
Support a plastic cup in the 250 cm$^3$ beaker.
Use the measuring cylinder to transfer 25 cm$^3$ of FA 6 into the plastic cup.
Record the initial temperature of the acid.
Weigh the container with FA 4 and record the mass.
Slowly add all the FA 4 to the acid in the plastic cup. Note that the reaction will be vigorous. Avoid inhaling any acid spray produced.
Stir constantly until the maximum temperature is reached. Record the maximum temperature.
Weigh the container from which you added FA 4 and record the mass.
Calculate and record the mass of FA 4 used and the temperature rise.
(b) Calculation
(i) Calculate the energy produced during this reaction.
[Assume that 4.2 J are needed to raise the temperature of 1.0 cm$^3$ of solution by 1.0 °C.]
energy produced = ....................... J
(ii) Calculate the number of moles of anhydrous sodium carbonate used.
[A$_r$: C, 12.0; O, 16.0; Na, 23.0]
moles of $\text{Na}_2\text{CO}_3$ = ................... mol
(iii) Calculate the enthalpy change, in kJ mol$^{-1}$, for the reaction below.
$$\text{Na}_2\text{CO}_3(s) + 2\text{HCl}(aq) \rightarrow 2\text{NaCl}(aq) + \text{CO}_2(g) + \text{H}_2\text{O}(l)$$
enthalpy change for anhydrous $\text{Na}_2\text{CO}_3$ = .......... ............ kJ mol$^{-1}$
(c) Determination of the enthalpy change for the reaction of hydrated sodium carbonate, FA 5, with excess hydrochloric acid.
**Method**
Support the second plastic cup in the 250 cm$^3$ beaker.
Use the measuring cylinder to transfer 25 cm$^3$ of FA 6 into the plastic cup.
Record the initial temperature of the acid.
Weigh the container with FA 5 and record the mass.
Slowly add all the FA 5 to the acid in the plastic cup. Note that the reaction will be vigorous. Avoid inhaling any acid spray produced.
Stir constantly until the minimum temperature is reached. Record the minimum temperature.
Weigh the container from which you added FA 5 and record the mass.
Calculate and record the mass of FA 5 used and the temperature decrease.
(d) Calculation
(i) Calculate the energy absorbed during this reaction.
[Assume that 4.2 J are absorbed when the temperature of 1.0 cm$^3$ of solution falls by 1.0 °C.]
energy absorbed = ....................... J
(ii) Use your answer to 1(c)(vi) to calculate the number of moles of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$, used.
[A$_r$: H, 1.0; C, 12.0; O, 16.0; Na, 23.0]
(If you were unable to calculate x in 1(c)(vi), you may assume that x = 8. Note: this is not the correct value.)
moles of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ = ................... mol
(iii) Calculate the enthalpy change, in kJ mol$^{-1}$, for the reaction below.
$$\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}(s) + 2\text{HCl}(aq) \rightarrow 2\text{NaCl}(aq) + \text{CO}_2(g) + (x + 1)\text{H}_2\text{O}(l)$$
enthalpy change for $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ = .......... ............ kJ mol$^{-1}$
(e) Use your values for the enthalpy changes calculated in (b)(iii) and (d)(iii) to calculate the enthalpy change for the reaction below.
$$\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}(s) \rightarrow \text{Na}_2\text{CO}_3(s) + x\text{H}_2\text{O}(l)$$
Show clearly how you obtained your answer.
(If you were unable to calculate the enthalpy changes, you should assume that the value in (b)(iii) is $-$43.1 kJ mol$^{-1}$ and the value in (d)(iii) is +32.8 kJ mol$^{-1}$. Note: these are not the correct values.)
enthalpy change = .......... ............ kJ mol$^{-1}$
(f) A student suggested that the experiments carried out in (a) and (c) would be more accurate if double the volume of hydrochloric acid had been used, but with hydrochloric acid of concentration 2.0 mol dm$^{-3}$, instead of 4.0 mol dm$^{-3}$.
State and explain whether or not you agree with the student’s suggestion.
501
556
561
(a) FA 7 and FA 8 are mixtures. Each mixture contains one cation and two anions from those listed on pages 12 and 13. Carry out the following tests and record all your observations in the table below.
[Table with tests and observations for FA 7 and FA 8]
(i) Place a spatula measure of FA 7 or FA 8 into a boiling tube, then add approximately 5 cm3 of dilute nitric acid using the measuring cylinder.
(Do not use FA 2 as the dilute nitric acid.)
Then
add about 5 cm3 of distilled water and shake to mix the solution.
Use a 1 cm depth of the solution obtained in a test-tube for each of tests (ii)–(v).
(ii) Add aqueous sodium hydroxide.
(iii) Add aqueous ammonia.
(iv) Add aqueous barium nitrate followed by dilute nitric acid.
(v) Add aqueous silver nitrate followed by aqueous ammonia.
(vi) Use your observations to identify the following ions.
FA 7 contains cation .........................................
FA 7 contains anions ......................................... and .......................................................
FA 8 contains anions ......................................... and .......................................................
State and explain all the evidence for your identification of the cation in FA 7.
....................................................................................................................................................
....................................................................................................................................................
....................................................................................................................................................
[7]
(b) FA 9 contains two cations from the list on page 12.
(i) Transfer approximately half of the FA 9 into a hard-glass test-tube and heat gently at first, and then strongly, until no further change is seen.
Test with litmus papers while you are heating.
Record all your observations below.
....................................................................................................................................................
....................................................................................................................................................
....................................................................................................................................................
....................................................................................................................................................
....................................................................................................................................................
(ii) Carry out further tests that will enable you to identify both cations in FA 9.
Describe your tests briefly and state your observations.
FA 9 contains cations ........................................... and ......................................................
[5]
521
535
544
