Practicals

Question 1

How would you prepare a standard solution. (4)

Answer 1

• Weigh mass of compound
• Dissolve in water in a beaker then add to volumetric flask
• Make the solution up to 250cm3 with the bottom of the meniscus on the graduation line
• Place stopper on the flask and invert to mix

Question 2

Describe the steps in obtaining a pure sample of cyclohexane from a distilled mixture. (4)

Answer 2

• Place mixture in separating funnel and add an equal volume of concentrated NaCl.
• Remove the more dense layer on the bottom by running off the tap. The less dense layer is on the top and is poured into a beaker.
• Dry organic layer with anhydrous salt; eg MgSO4, CaCl2. This removes any excess water left behind.
• Redistill organic layer as there can still be impurities. The more narrow the boiling points, the purer the product.

Question 3

How do you prepare a burette or pipette before use in a titration.

Answer 3

• Rinse with the solution that the apparatus is holding before use
• eg in a pipette, rinse with alkaline solution,
  With a burette rinse with acid.

Question 4

Describe the experiment to identify an unknown alcohol (3)
Include equipment (4)

Answer 4

• Equipment used; reflux apparatus: pear shape flask, condenser, rubber tubing, stand and clamp.
• Kr2Cr2O7 and H2SO4 and excess alcohol placed in pear flask and heated under the mantle
• Primary and secondary alcohol turn from orange to green
• Tertiary alcohol is not oxidised so it stays orange.

Question 5

Describe the distillation process to form an aldehyde (4)

Include equipment used (8)

Answer 5

Equipment used: Pear shape flask, condenser, still head, receiver, thermometer, screw tap adapter, rubber tubing, stand and clamp.

• Kr2Cr2O7 and H2SO4 placed in pear flask with excess alcohol.
• Thermometer used to ensure temperature is below 100.
• Aldehyde evaporates first due to the lowest boiling point.
• condensed and collected through the receiver.

Question 6

Describe the experiment to produce pure sample of cyclohexene from cyclohexanol. (9)

Answer 6

• Distillation apparatus set up; pear shape flask, condenser, receiver, thermometer etc.
• Add known volume of cyclohexanol.
• Heat cyclohexanol with H3PO4 under 100 degrees and collect product via receiver.
• Decant any anti-bumping granules present
• Add collected product into separating funnel.
• Invert the funnel a few times and wait for distinct layers to form. Run off the aqueous layer at the bottom.
• Add sodium carbonate to the organic layer until effervescence stops. Allow two layers to form and remove aqueous layer and discard it.
• Dry organic layer with a solid, anhydrous salt- Magnesium sulfate. Filter and remove anhydrous salt after use
• Distill again.

Question 7

How can rinsing the pipette affect the titre in a titration?

Answer 7

• Smaller titre; less moles of the solution when transferred to conical flask
• Less moles needed to neutralise the solution.

Question 8

How can rinsing the burette affect the titre in a titration? (2)

Answer 8

• Water left in the burette means that there is less moles of that solution
• Causes a larger titre as more moles are needed to neutralise solution inside the conical flask.

Question 9

How can rinsing the conical flask affect the titre in a titration?

Answer 9

• No change occurs

Question 10

Describe how you would purify a liquid that is miscible with water.

Answer 10

1. Allow the mixture to cool first.
2. Decant any anti-bumping granules if the liquid is organic and the reaction involved a Quickfit apparatus.
3. If acid is present, add Na2CO3 (sodium carbonate) until effervescence stops.
4. Redistil to complete purification.

Question 11

Describe how you would purify a solid product.

Answer 11

1. Allow the reaction mixture to cool.
2. Filter mixture under REDUCED pressure and leave impure product to dry.

Recrystallisation:

3. Transfer impure solid into a boiling tube
4. Dissolve in the minimum volume of hot solvent
5. Filter the hot solution
6. All filtrate to cool at room temperature
7. Place filtrate in an ice bath- crystals will start to form.
8. Filter under REDUCED pressure again. Rinse with cold water and dry.
9. Recrystallise and filter under reduced pressure again.

Question 12

Describe how you would obtain the melting point of a solid.

Answer 12

Using a Thiele tube:

1. Take a glass capillary tube, hold the end and rotate on a hot flame from a Bunsen burner. Do so until the tube is sealed.
2. Leave tube to cool.
3. Fill tube with sample, to around 3mm. Ensure the sample is dry and free flowing.
4. Set up the Thiele tube and attach the capillary tube, containing the sample, with a rubber band.
5. Insert the thermometer into the hole of the cork, ensuring the tip is touch the liquid.
6. Heat the side arm of the Thiele tube SLOWLY with a micro-burner and observe the solid.
7. Stop heating when the solid starts melting and take out the thermometer immediately, to record the temperature.

Repeat the experiment a few times to get a more precise reading.

Question 13

Describe the method to identify an unknown Group 2 metal.

Answer 13

Equipment:
Conical flask
Bung
Delivery tube
Gas syringe
Balance

1. Weigh a sample of the unknown metal with a balance.
2. Add 25cm3 of 1mol dm-3 of HCl to the conical flask.
3. Add metal to the acid and quickly placed the bung.
4. Measure the volume of H2 collected in the gas syringe/ Take measurement when no more gas is produced.

Moles of H2 = moles of metal
Mr of metal = mass of metal/ moles of metal

Question 14

Describe how you would carry out an acid-base titration.

Answer 14

1. Measure the volume of one solution (i.e acid) using a pipette and place in a conical flask. Add a few drops of indicator into conical flask.
2. Add the other solution (i.e alkali) into burette and record the volume to the nearest 0.05cm3.
3. Run the solution in the burette into the conical flask, whilst swirling the conical flask. The indicator will change colour at the end point. Stop running the solution from the burette.
4. Record the end volume in the burette and calculate the titre: initial volume- final volume.
5. Repeat experiment until at least 2 titres are concordant- agree to within 0.10 cm3

Question 15

Describe how you can calculate the initial rate of reaction in a reaction where gas is produced.

Answer 15

Equipment:
Conical flask
Delivery tube
Gas syringe
Bung
Stopwatch
Balance

1. Reactants are added to conical flask with bung placed quickly. A stopwatch is started.
2. The volume of gas produced is recorded at regular intervals, like 20 secs.
3. Reaction is complete when no more gas is produced.
4. A graph is plotted to show time against volume of gas produced.
5. Initial rate is calculated by drawing a tangent at t=0.

Question 16

Describe the experiment used to determine Kc.

Answer 16

1. In a conical flask mixed known concentrations and volumes of reactants and catalyst.
2. Add known volume and concentration of acid catalyst into a separate conical flask as a control.
3. Place a stopper on both flasks and leave for a week to reach equilibrium.
4. Carry out a titration on equilibrium using a standard solution of NaOH.
5. Repeat the titration with acid control to calculate the amount of acid used.