Chemistry · Chapter 29
Study notes aligned to the official NEB syllabus.
Before you start, open your own school practical record book and check the exact list of experiments and the mark weighting your school follows, because the prescribed list is issued to schools and the small print varies slightly from one institution to another. The experiments written out below are the ones that make up the core of Grade 12 chemistry laboratory work in Nepal, and they are the ones that examiners return to year after year: a redox titration with potassium permanganate, the standardisation of that permanganate solution, the sodium fusion test for elements in an organic compound, the functional group tests, and the systematic analysis of an inorganic salt. Learn these thoroughly and you will be ready for almost anything the practical examination puts in front of you, but confirm the list itself against your record book.
The practical examination is usually assessed on four things at once, so treat all four as marks. First, the bench work: whether you set up the apparatus correctly, rinse and fill the burette properly, and reach the end point cleanly. Second, the record: a ruled observation table with proper column headings, readings written to the correct number of decimal places, and a calculation that carries units through to the answer. Third, the result and the honesty of it, because a titration copied from a friend is usually obvious from impossible concordance. Fourth, the viva, in which the examiner asks why you did each thing rather than what you did. Laboratory discipline underpins all of it. Wear your apron and, where the experiment involves concentrated acid, eye protection. Never pipette any solution by mouth, use a pipette filler. Read a burette with your eye level with the meniscus, taking the lower meniscus for colourless solutions and the upper meniscus for the deeply coloured permanganate. Add concentrated acid to water and never the reverse. Work with volatile and foul smelling reagents such as chloroform, carbon disulphide and the isocyanide from the carbylamine test in a fume cupboard or at least near an open window. Label every test tube, keep the bench dry, and return stock bottles to their shelf with the correct stopper. Write your observations at the bench as you make them, not afterwards from memory.
To prepare 250 mL of a standard solution of oxalic acid and to use it to determine the strength of a given solution of potassium permanganate by redox titration.
A chemical balance with a weight box, a watch glass or weighing bottle, a 250 mL volumetric (measuring) flask with its stopper, a funnel, a glass rod, a wash bottle of distilled water, a 50 mL burette with a burette stand and clamp, a white glazed tile, a 20 mL or 25 mL pipette with a filler, a 250 mL conical flask, a beaker, a wire gauze and burner or a hot plate, and a thermometer. Chemicals: oxalic acid crystals, that is hydrated oxalic acid $H_2C_2O_4 \cdot 2H_2O$, the given potassium permanganate solution, and dilute sulphuric acid, about 1 M.
Oxalic acid dihydrate is a primary standard: it is available pure, it is not hygroscopic, and its formula mass of 126 is known exactly, so a solution of accurately known concentration can be made simply by weighing. Potassium permanganate is not a primary standard, because the solid always carries a little manganese dioxide, and the solution is slowly reduced by dust, light and traces of organic matter. Its strength must therefore be found against something that is a primary standard, and that is what this titration does. In acidic medium the permanganate ion is reduced from the +7 to the +2 oxidation state, gaining five electrons, while each oxalate ion loses two electrons and leaves as carbon dioxide.
$$ \begin{aligned} MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O \ C_2O_4^{2-} \rightarrow 2CO_2 + 2e^- \ 2MnO_4^- + 5C_2O_4^{2-} + 16H^+ \rightarrow 2Mn^{2+} + 10CO_2 + 8H_2O \ 2KMnO_4 + 5H_2C_2O_4 + 3H_2SO_4 \rightarrow K_2SO_4 + 2MnSO_4 + 10CO_2 + 8H_2O \end{aligned} $$
Since permanganate gains five electrons, its equivalent mass in acidic medium is $158/5 = 31.6$, and the equivalent mass of hydrated oxalic acid is $126/2 = 63$. Dissolving 1.575 g in 250 mL therefore gives a 0.1 N solution. The calculation at the end uses
$$N_1 V_1 = N_2 V_2$$
where subscript 1 refers to the permanganate in the burette and subscript 2 to the oxalic acid in the flask.
Rule a table with five columns headed Number of observation, Volume of oxalic acid taken in mL, Initial burette reading in mL, Final burette reading in mL, and Volume of $KMnO_4$ used in mL, and allow at least four rows. Record every burette reading to two decimal places, the second decimal being 0 or 5 since the burette is graduated in 0.1 mL. Concordant readings are two or preferably three consecutive titre values agreeing within 0.1 mL of one another, and only concordant readings may be averaged; the rough first titration is recorded but excluded from the mean. Note the colour change at the end point as colourless to permanent pale pink.
The strength of the given potassium permanganate solution, calculated from $N_1 V_1 = N_2 V_2$ using the mean concordant titre, is reported in normality, and then in grams per litre by multiplying the normality by 31.6, and in molarity by dividing the normality by 5.
Weigh the oxalic acid quickly, keep the volumetric flask stoppered, and never heat a volumetric flask. Use dilute sulphuric acid only: hydrochloric acid would itself be oxidised to chlorine and give a falsely high titre, and nitric acid is an oxidising agent in its own right. Add the acid before titrating and not afterwards. Keep the temperature in the stated range, because below about 60 degrees the reaction is too slow to give a sharp end point and above boiling the oxalic acid decomposes. Do not use an external indicator. Wipe the burette jet and make sure no drop hangs from it before taking a final reading, and do not let the flask contents splash out while swirling.
Before you start, open your own school practical record book and check the exact list of experiments and the mark weighting your school follows, because the prescribed list is issued to schools and the small print varies slightly from one institution to another. The experiments written out below are the ones that make up the core of Grade 12 chemistry laboratory work in Nepal, and they are the ones that examiners return to year after year: a redox titration with potassium permanganate, the standardisation of that permanganate solution, the sodium fusion test for elements in an organic compound, the functional group tests, and the systematic analysis of an inorganic salt. Learn these thoroughly and you will be ready for almost anything the practical examination puts in front of you, but confirm the list itself against your record book.
The practical examination is usually assessed on four things at once, so treat all four as marks. First, the bench work: whether you set up the apparatus correctly, rinse and fill the burette properly, and reach the end point cleanly. Second, the record: a ruled observation table with proper column headings, readings written to the correct number of decimal places, and a calculation that carries units through to the answer. Third, the result and the honesty of it, because a titration copied from a friend is usually obvious from impossible concordance. Fourth, the viva, in which the examiner asks why you did each thing rather than what you did. Laboratory discipline underpins all of it. Wear your apron and, where the experiment involves concentrated acid, eye protection. Never pipette any solution by mouth, use a pipette filler. Read a burette with your eye level with the meniscus, taking the lower meniscus for colourless solutions and the upper meniscus for the deeply coloured permanganate. Add concentrated acid to water and never the reverse. Work with volatile and foul smelling reagents such as chloroform, carbon disulphide and the isocyanide from the carbylamine test in a fume cupboard or at least near an open window. Label every test tube, keep the bench dry, and return stock bottles to their shelf with the correct stopper. Write your observations at the bench as you make them, not afterwards from memory.
To prepare 250 mL of a standard solution of oxalic acid and to use it to determine the strength of a given solution of potassium permanganate by redox titration.
A chemical balance with a weight box, a watch glass or weighing bottle, a 250 mL volumetric (measuring) flask with its stopper, a funnel, a glass rod, a wash bottle of distilled water, a 50 mL burette with a burette stand and clamp, a white glazed tile, a 20 mL or 25 mL pipette with a filler, a 250 mL conical flask, a beaker, a wire gauze and burner or a hot plate, and a thermometer. Chemicals: oxalic acid crystals, that is hydrated oxalic acid , the given potassium permanganate solution, and dilute sulphuric acid, about 1 M.
Oxalic acid dihydrate is a primary standard: it is available pure, it is not hygroscopic, and its formula mass of 126 is known exactly, so a solution of accurately known concentration can be made simply by weighing. Potassium permanganate is not a primary standard, because the solid always carries a little manganese dioxide, and the solution is slowly reduced by dust, light and traces of organic matter. Its strength must therefore be found against something that is a primary standard, and that is what this titration does. In acidic medium the permanganate ion is reduced from the +7 to the +2 oxidation state, gaining five electrons, while each oxalate ion loses two electrons and leaves as carbon dioxide.
Since permanganate gains five electrons, its equivalent mass in acidic medium is , and the equivalent mass of hydrated oxalic acid is . Dissolving 1.575 g in 250 mL therefore gives a 0.1 N solution. The calculation at the end uses
where subscript 1 refers to the permanganate in the burette and subscript 2 to the oxalic acid in the flask.
Rule a table with five columns headed Number of observation, Volume of oxalic acid taken in mL, Initial burette reading in mL, Final burette reading in mL, and Volume of used in mL, and allow at least four rows. Record every burette reading to two decimal places, the second decimal being 0 or 5 since the burette is graduated in 0.1 mL. Concordant readings are two or preferably three consecutive titre values agreeing within 0.1 mL of one another, and only concordant readings may be averaged; the rough first titration is recorded but excluded from the mean. Note the colour change at the end point as colourless to permanent pale pink.
The strength of the given potassium permanganate solution, calculated from using the mean concordant titre, is reported in normality, and then in grams per litre by multiplying the normality by 31.6, and in molarity by dividing the normality by 5.
Weigh the oxalic acid quickly, keep the volumetric flask stoppered, and never heat a volumetric flask. Use dilute sulphuric acid only: hydrochloric acid would itself be oxidised to chlorine and give a falsely high titre, and nitric acid is an oxidising agent in its own right. Add the acid before titrating and not afterwards. Keep the temperature in the stated range, because below about 60 degrees the reaction is too slow to give a sharp end point and above boiling the oxalic acid decomposes. Do not use an external indicator. Wipe the burette jet and make sure no drop hangs from it before taking a final reading, and do not let the flask contents splash out while swirling.