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prac_wksheet_5_grade_11 by stariya

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PRACTICAL WORK SHEET 5                                                                          GRADE 11

                              PREPARATION OF A STANDARD SOLUTION

AIM

To prepare a 0,1 mol.dm-3 oxalic acid solution.

INSTRUCTIONS

1.       This experiment may be carried out individually or in groups. Each member of the group must
         do a complete report on the experiment.
2.       You will be assessed on the following skills:
        Investigation
        Manipulative / Procedural
        Interpretation of data
3.       Make sure that you take note of all the criteria that are required for the specific task.
4.       Follow the procedure accurately and number all tasks properly.
5.       Answer the questions clearly.

PROCEDURE

1.       Six salts will be given to you. Complete the following table:

                                                          Number of H2O
                                     Formula of the     molecules attached        Formula of the
             Compound
                                      compound          to one molecule of         crystal / salt
                                                          the compound
          Sodium sulphate
          Sodium carbonate
           Copper sulphate
         Magnesium sulphate
             Oxalic acid
             Citric acid

                                           ASSESSMENT OF TABLE
Criteria:
*       Correct formulae for the compounds.
*       Number of H2O molecules attached to one molecule of the compound.
*       Correct formula for the crystal.

                             Skill          Level     Mark          Level Descriptor
                                              1        1      1 crystal with correct formula.
                                              2        2     2 crystals with correct formulae.
                             3
                                              3        3     3 crystals with correct formulae.
                        Investigating
                                              4        4     4 crystals with correct formulae.
                      (gathering data)
                                              5        5     5 crystals with correct formulae.
                                              6        6     6 crystals with correct formulae.
                                                                                      Maximum marks: 6

2.       Calculate the molar mass of the oxalic acid compound. Write down the correct SI–unit.
3.       Calculate the molar mass of the oxalic acid crystal. Write down the correct SI–unit.
4.       How much oxalic acid crystals (mass) are needed to be dissolved in a 200 ml
         volumetric flask, in order to make up a solution with a concentration of 0,1 mol.dm-3?
         In the calculation, cancellation of units in the substitution steps must be indicated in order to

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        get to the final unit of the answer.
5.      Determine the mass of  2,25g oxalic acid crystals in a 250 ml beaker. Write down the
        exact mass of acid taken.
6.      Completely dissolve the crystals in  100 ml distilled water.
7.      Use a funnel to pour the solution into the 200 ml beaker. Use a washing bottle to transfer all
        the dissolved salt to the volumetric flask.
8.      Fill the 200 ml volumetric flask with distilled water up to the volume mark and shake well.
        Label the flask properly indicating the content, concentration and date of preparation.

                          ASSESSMENT OF THE USE OF APPARATUS
Criteria:
 Crystal properly dissolved.
 Use of funnel.
 Accurate transfer of liquid.
 Flask filled to the mark (bottom of meniscus).
 Flask properly labelled.

                   Skill                   Level     Mark             Level Descriptor
                                                               Procedure meets two of the above
                                            1         1
                                                                           criteria.
                                                              Procedure meets three of the above
                   6                        2         2
                                                                           criteria.
       Manipulative / Procedural
                                                              Procedure meets four of the above
         (use of apparatus)                 3         3
                                                                           criteria.
                                                               Procedure meets five of the above
                                            4         4
                                                                           criteria.
                                                                                     Maximum marks: 4

9.      Calculate the exact concentration of the solution by using the
        weighed mass. Show the units in the substitution step and cancel the units
        to derive the unit of the final answer.


                                ASSESSMENT OF CALCULATION
Criteria:
 Show correct formula.                                                                  200 cm3
 Correct substitution of values.                                                         20° C
 Correct substitution of SI-units.
 Correct answer with unit .

                     Skill                   Level    Mark              Level Descriptor
                                                               Calculation meets one the above
                                                1         1
                                                                            criteria.
                                                              Calculation meets two of the above
                     5                          2         2
                                                                            criteria.
          Interpretation of data
                                                              Calculation meets three of the above
        (use of ata & calculations)             3         3
                                                                            criteria.
                                                               Calculation meets all of the above
                                                4         4
                                                                            criteria.
                                                                                    Maximum marks: 4




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QUESTIONS

1.      Does the mole quantity of the acid change if the beaker is rinsed and/or water is added to the
        oxalic acid?
2.      Can this solution be regarded as a standard solution?
3.      What is the concentration (mol.dm-3) of a NaOH solution made by using 2g NaOH in
        100ml solution?
4.      How many grams of KOH must be dissolved in water to prepare a 200ml KOH solution of
        concentration 0,3 mol.dm-3?
5.      What mass of oxalic acid is needed to prepare a 250ml solution of 0,75 mol.dm-3
        concentration?
6.      25 ml of the 0,75 mol.dm-3 oxalic acid is pipetted into an Erlenmeyer flask. 150 ml water is
        added to the oxalic acid in the flask. What is the number of moles of oxalic acid present in the
        flask after the addition of water?




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