Practical Session: 2

Question 1

What is the title of practical 2?

Answer 1

Titration Analysis of Vinegar.

Question 2

What are the aims of lab 2?

Answer 2

1. Strengthen teaching o mole calculations and volumetric analysis.
2. Show off characteristics of acid-base titrations.
3. Show off correct use of paired t-tests.

Question 3

What is an important part of analysis?

Answer 3

Quantification.

Question 4

How can we quantify?

Answer 4

Comparing standards.

Like Beer Lambert relationship.

Question 5

How can we define quantities?

Answer 5

With stoichiometric reaction with clear end point.

React a known amount of reagent with unknown material –> calculate unknown.

Question 6

How is the quantification called as an experiment?

Answer 6

Titration.

Question 7

What happens in titration?

Answer 7

Have: known volume of unknown concentration.
Add: measured volume of known concentration.

Question 8

How is the unknown concentration measured?

Answer 8

By pipette.

Question 9

How is known concentration measured?

Answer 9

By burette.

Question 10

How can we identify the end point?

Answer 10

By a colour change.

Physical change = ion concentration.

Question 11

Why does the vinegar need to be diluted?

Answer 11

To avoid excessive reagent use.

To improve end point.

Question 12

What will undiluted vinegar do?

Answer 12

Mask indicator colour change.

Question 13

What will we use with diluted vinegar for a colour change?

Answer 13

Indicator.

Question 14

Why do we use pH meters?

Answer 14

To show off their use.

To compare them with indicator.

Question 15

Why do we avoid putting alkali in burette?

Answer 15

It reacted with grease.

It blocks burette if it dries out.

Question 16

Where does alkali go in this practical?

Answer 16

In burette.

Initially.

Question 17

What do we must do to the alkali before doing the back titration?

Answer 17

Wash it out.

Rinse it with acid.

Question 18

What is the Method for the experiment of ‘Forward Titration’?

Answer 18

1. Pipette 2mL of vinegar and transfer to a 100mL volumetric flask. Make up to the mark with distilled water (Solution A).
2. Pipette 10.0 ml of this diluted vinegar (solution A) into a 100ml beaker. Place a magnet in the flask and place on a magnetic stirrer (This will stir your solution for you while you titrate).
3. Insert your pH probe and check that it is covered by the solution. If not, add distilled water to the mixture. (This will dilute the solution and slightly affect the pH but will not affect the number of moles present).
4. Fill the burette with 0.02 M NaOH and titrate, taking pH readings every 0.5mL up to 15mL. Take readings every 0.1 mL when the pH is changing more rapidly (usually about pH 5 or 6). Note: do not stir the solution with the pH meter.
5. Whilst you are taking your readings plot a graph of pH (vertical axis) against volume added. You will need a range of 2-12 for pH and 0-15mL for volume.
6. Still using NaOH in the burette and using the indicator phenolphthalein (2 drops), carry out titrations of 10.0 mL diluted vinegar (solution A, conical flask). You need to carry out a minimum of two duplicate concurrent (within 0.1 mL) titrations. (bromomthymol blue [yellow in acid, deep blue in alkali]).
7. Calculate the concentration of vinegar in your original sample for both titration methods. (NB don’t forget the initial dilution)
   CH3COOH + NaOH  NaCH3COO + H2O
   (M_1×V_1)/N_1 = (M_2×V_2)/N_2

Question 19

What is the method of the experiment of ‘Back Titration’?

Answer 19

8. Pipette 10ml of diluted vinegar (solution A) into a 100ml beaker.
9. Using a burette add 15 mL of 0.02 M NaOH.
10. This time using phenolphthalein as an indicator (2 drops) carry out a back titration with 0.02M HCl. You need to carry out a minimum of two duplicate concurrent (within 0.1 mL) titrations. Note: this time the colour change is pink to colourless. Take care when approaching the end point. you may accidentally overshoot the end point in the first titration.
11. Calculate the molarity of the original vinegar by back titration.
    NaOH + HCl  NaCl + H2O
12. Record all your data and calculations in your lab notebook.
13. Calculate the mean ( SD) of your results from the forward and back titrations.
14. You will need to enter your data on the spreadsheet at the front of the class.
15. Use the pooled data to carry out a paired T-test analysis of the results (this will be covered in the statistics sessions)