Manipulation, Measurement and Observation

Method

• Fill the burette with FA 4.
• Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
• Use the measuring cylinder to add approximately $10\text{ cm}^3$ of FA 2 to the same conical flask.
• Use the measuring cylinder to add approximately $20\text{ cm}^3$ of FA 3 to the mixture in the conical flask. The mixture will now be a brown colour, due to iodine produced in the reaction.
• Begin your rough titration by adding FA 4 from the burette until the mixture becomes light brown.
• Add 10 drops of starch indicator. The mixture will become darker.
• Continue titrating until the mixture becomes an off-white colour. This is the end-point.
• Add one drop of starch indicator to check that no traces of dark colour are produced. If the mixture stays off-white, the titration is finished. If some dark colour is produced, because iodine is still present, continue the titration.
• Record your burette readings and the rough titre in the space below.

The rough titre is ............................. $\text{cm}^3$.

• 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 4 added in each accurate titration.

Keep FA 3 and starch indicator for use in Question 3.

From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations.
Show clearly how you obtained this value.

The iodine produced required ............................. $\text{cm}^3$ of FA 4.

Method

Read through the method before starting your practical work and prepare a table below for recording your results.

Experiment 1

• Place the plastic cup in the $250\text{ cm}^3$ beaker.
• Pour $25\text{ cm}^3$ of FB 3 into the larger measuring cylinder.
• Measure and record the temperature of FB 3.
• Rinse and dry the thermometer.
• Use the smaller measuring cylinder to transfer $25\text{ cm}^3$ of FB 4 into the plastic cup.
• Measure and record the temperature of FB 4.
• Add the $25\text{ cm}^3$ of FB 3 to FB 4 in the plastic cup and stir the mixture. Measure and record the highest temperature reached.
• Calculate and record the average initial temperature of the solutions.
• Calculate and record the temperature rise.
• Empty the plastic cup, rinse it with water and shake it to remove excess water.

Experiment 2

• Repeat the method given for Experiment 1 using $50\text{ cm}^3$ of each solution.
• Use the larger measuring cylinder for FB 3 and the smaller measuring cylinder for FB 4.

Results

Show by calculation that in Experiment 1, the number of moles of acid was in excess of the number of moles of sodium hydroxide.

Calculate the heat energy evolved in Experiment 1.
[Assume that $4.2\text{ J}$ of heat energy changes the temperature of $1.0\text{ cm}^3$ of solution by $1.0\text{ }^\circ\text{C}$.]

heat energy evolved = ............................ J

Calculate the enthalpy change, in $\text{kJ mol}^{-1}$, for Experiment 1.

enthalpy change = ...... ............................. $\text{kJ mol}^{-1}$
(sign) (value)

Calculate the number of moles of sodium hydroxide neutralised in Experiment 2.

moles of $\text{NaOH}$ = ............................ mol

Calculate the enthalpy change, in $\text{kJ mol}^{-1}$, for Experiment 2.

enthalpy change = ...... ............................. $\text{kJ mol}^{-1}$
(sign) (value)

Method

Preparing a solution of FA 2

• Weigh the stoppered container of FA 2. Record the mass in the space below.
• Tip all the FA 2 into the beaker.
• Reweigh the container with its stopper. Record the mass.
• Calculate and record the mass of FA 2 used.
• Add approximately $100\text{ cm}^3$ of distilled water to the FA 2 in the beaker.
• Stir the mixture with a glass rod until all the FA 2 has dissolved.
• Transfer this solution into the $250\text{ cm}^3$ volumetric flask.
• Wash the beaker with distilled water and transfer the washings to the volumetric flask.
• Rinse the glass rod with distilled water and transfer the washings to the volumetric flask.
• Make up the solution in the volumetric flask to the mark using distilled water.
• Shake the flask thoroughly.
• This solution of impure $\text{NaHCO}_3$ is FA 3. Label the flask FA 3.

Results

Titration

• Fill the burette with FA 1.
• Pipette $25.0\text{ cm}^3$ of FA 3 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 ............................. $\text{cm}^3$.

• 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 1 added in each accurate titration.

Keep FA 1 for use in Question 2.

From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations.
Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FA 3 required ............................. $\text{cm}^3$ of FA 1.

Method

• Fill the burette with FA 1.
• Pipette $25.0\text{ cm}^3$ of FA 4 into a conical flask.
• Use the measuring cylinder to add $10\text{ cm}^3$ of FA 2 to the same conical flask.
• Use the measuring cylinder to add $10\text{ cm}^3$ of FA 3 to the same conical flask. The mixture will become brown because of the formation of $\text{I}_2$, and will become cloudy because of the formation of the white precipitate of $\text{CuI}$.
• Add FA 1 from the burette until the mixture becomes a light brown colour.
• Add 10 to 20 drops of starch indicator until the mixture becomes blue-black.
• Continue to titrate with FA 1 until the blue-black colour disappears leaving a mixture with an off-white solid. This is the end-point.
• You should test that the end-point has been reached by adding 2 more drops of starch indicator. If the titration has reached the end-point the added starch indicator will cause no change in colour.
• Perform a rough titration and record your burette readings in the space below.

The rough titre is ............................ $\text{cm}^3$.

• 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 1 added in each accurate titration.

From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations.
Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FA 4 required ............................. $\text{cm}^3$ of FA 1.

Method

Before starting any practical work, read through all the instructions and prepare a table for your results in the space provided.

• Weigh the crucible with a lid and record the mass.
• Add all the FA 5 to the crucible.
• Reweigh the crucible with the lid and FA 5. Record the mass.
• Place the crucible in the pipe-clay triangle on top of a tripod.
• Heat the crucible gently with the lid on for about 1 minute.
• Remove the lid and then heat more strongly for a further 4 minutes.
• Replace the lid and allow the crucible to cool.
• While the crucible is cooling you may wish to begin work on Question 3.
• Once the crucible has cooled, reweigh the crucible with the lid and contents. Record the mass.
• Calculate and record the mass of FA 5 used, the mass of the residue and the mass of water lost.

Starting with the same mass of hydrated barium chloride, suggest how you could modify the experiment to determine more accurately the mass of water lost.

Method

Dilution

• Transfer all the reaction mixture that you prepared in 1(a) from the $250\text{ cm}^3$ beaker to the $250\text{ cm}^3$ volumetric flask.
• Rinse the beaker with a little distilled water and add these washings to the volumetric flask.
• Fill the volumetric flask to the line with distilled water. Stopper the flask and shake it to ensure thorough mixing.
• Label this solution FA 4.

Titration

• Fill the burette with FA 4.
• Use a pipette to transfer $25.0\text{ cm}^3$ of FA 3 into a conical flask.
• Add a few drops of methyl orange.
• Perform a rough titration and record your burette readings in the space below.

The rough titre is .................... $\text{cm}^3$.

• 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 4 added in each accurate titration.

From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FA 3 required ....................... $\text{cm}^3$ of FA 4.

One of the sources of error in determining the concentration of FA 2 involves measuring volumes of solutions in both Questions 1 and 2.

State which volume of solution that you have measured has the greatest percentage error. How could you have reduced this error?

Method

• Pipette $25.0\text{ cm}^3$ of FA 2 into the $250\text{ cm}^3$ volumetric flask. Keep remaining FA 2 for use in Question 2.
• Add distilled water to make $250\text{ cm}^3$ of solution and shake the flask thoroughly. Label this solution FA 3.
• Fill the burette with FA 3.
• Use the second pipette to transfer $25.0\text{ cm}^3$ of FA 1 into a conical flask.
• Add about 10 drops of bromophenol blue.
• Perform a rough titration and record your burette readings in the space below. The end point is reached when the solution becomes a permanent yellow colour.

The rough titre is ....................... $\text{cm}^3$.

• 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.

From your accurate titration results, obtain a suitable value for the volume of FA 3 to be used in your calculations. Show clearly how you obtained this value.

$25.0\text{ cm}^3$ of FA 1 required ....................... $\text{cm}^3$ of FA 3.