GRADE 12 PRESCRIBED EXPERIMENT 2: ACID-BASE REACTIONS 


                                 

WORK SHEET 
TOTAL MARK: 50

ACTIVITY

Titration of oxalic acid against sodium hydroxide to determine the concentration of the sodium hydroxide.

In this investigation you will prepare an acidic solution accurately and thus you will know its exact concentration. You will then react this acid with a base of an unknown concentration to determine the concentration of the base.

What you will need:

Erlenmeyer flasks,
Burettes,
Burette clamp,
Medicine  dropper,
Retort stand,
White tile /paper,
Measuring cylinders,
Mass meter
Oxalic acid,
Sodium Hydroxide,
Phenolphthalein as indicator,
Funnel,
Beaker,
Spatula,
Glass rod,
Pipette with sucker

What to do:

  1. Prepare a standard solution of oxalic acid which has a concentration of approximately 1mol.dm-3.
  2. Now prepare a sodium hydroxide solution by dissolving approximately 2g of dry sodium hydroxide in 500ml of water.
  3. Add two drops of the indicator solution.
  4. Place the burette in the clamp.
  5. Using the funnel, fill the burette to above zero mark with the acid solution.
  6. Then, holding the beaker, with which you used to pour the acid, beneath the burette, gradually open the tap.
  7. Allow the level of the base to come down to exactly zero (reading from the bottom of the meniscus).
  8. Pipette using the sucker exactly 25ml of oxalic acid solution in a volumetric flask.
  9. Add a few drops of phenolphthalein to the acid.
  10. Hold the conical flask beneath the burette with your right hand and gradually open the tap with your left.
  11. Swirl the conical flak continuously and watch it closely for the first sign of a colour change.
  12. As you see that you are approaching the point of neutralization, close the tap slightly so that you are adding drop by drop.
  13. When the colour changes completely the titration is finished.
  14. Close the tap and read from the burette how much acid was used.
  15. Repeat this procedure at least twice so that you have three readings for the volume of NaOH (of unknown concentration) required to neutralize exactly 25ml of oxalic acid (of known concentration).
  16. Take an average of these three and use it to calculate the concentration of the NaOH.
  17. Now calculate the concentration of the sodium hydroxide solution.
  18. Make a neat labeled sketch to represent the apparatus
  19. Now write a report using the format learnt in class.

     Questions

  1. What is the appropriate concentration of NAOH (2g in 500ml of water)
  2. Calculate the theoretical concentration of NaOH from the actual mass of NaOH you measured.
  3. How does your theoretical value for NaOH concentration (from the actual mass you measured) differ from the actual concentration you calculated (from the titration procedure)? Can you think of some reasons why your values may differ?

INTERMOLECULAR FORCES


Endothermic & Exothermic at the same time


Acid-Base Solutions


Electricity and Magnetism


Knowledge Area: Electricity and Magnetism

Topic: Internal resistance and Series-Parallel networks

Part 1

Aim

To determine the internal resistance of a battery

Precautions

Do not leave the switch closed at all times.  The battery will run flat. Close the switch only for taking the readings on the voltmeter and Ammeter.  Take your readings accurately. Do not conduct the experiment with wet hands.

Instructions

Place 4 x 1,5V cells in a cell holder in a series connection.  Connect a Switch, Ammeter and a Rheostat in series to the battery.  Connect the Voltmeter across the battery.

If a rheostat is not available, resistors or bulbs could be used in the place of a rheostat. The resistors or bulbs must be connected in series.  One resistor/bulb must be connected first and then the resistors/bulbs be increased to two in series, three in series, etc.

Draw the electrical circuit diagram. Set the Rheostat at 10W. Close the switch and reduce the resistance in the Rheostat step by step, increasing the current at the same time. Take the ammeter and voltmeter readings in each case/step. Open the switch while recording the readings in order to spare the batteries’ life span.

Take and record a minimum of 5 readings. Interpret and analyse the data in order to determine the internal resistance. Write a conclusion and prepare a report (write-up).

Part 2

Aim:

  1. To determine the equivalent resistance in a Series-parallel network electrical circuit.
  2. To compare the experimental values of the equivalent resistance to the theoretical values.

Instructions:

Place 4 x 1.5V cells in a cell in a cell holder in a series connection. Connect a voltmeter (V1) across the battery. Connect three resistors of different resistance. One resistor must be connected in series with a voltmeter (V2) across and the other two resistors must be connected in parallel with a voltmeter across (V3). Connect a switch and an ammeter.