Chemistry · Chapter 19
Study notes aligned to the official NEB syllabus.
Before you revise from this chapter, open your own school practical record book and check the exact list of experiments and the mark weighting your laboratory has been set, because the prescribed practical list varies slightly from school to school and is sometimes trimmed or reordered by the lab in charge. What follows is the practical work that is standard in Grade 11 chemistry laboratories in Nepal and that examiners routinely set: volumetric analysis by titration, qualitative analysis of acid and basic radicals, correct handling of volumetric glassware, and the determination of a physical constant such as a melting point.
The practical examination is held in your own laboratory, normally by an internal examiner working with an external examiner. You work alone at a bench under supervision, and the marks are divided between the write up you produce on the day, the accuracy of your result, your handling of apparatus, the record book you submit, and a short oral viva. Neatness and honesty carry real marks. Record every reading in ink at the moment you take it, in a table ruled up before you begin, and never nudge a reading to make the arithmetic tidy: titres that converge honestly to within 0.1 cm3 earn full credit, while identical readings in every trial only make an examiner suspicious.
Laboratory discipline is part of the assessment. Tie long hair back and wear an apron. Never draw liquid into a pipette by mouth; use a filler, which matters most with acids and with lead and barium salts, all of which are poisonous. When diluting, add concentrated acid to water and never water to concentrated acid, and smell a gas by wafting it towards you with your hand. Handle hydrogen sulphide, concentrated ammonia and concentrated acids in a fume cupboard or beside an open window. Keep the bench dry, label every test tube, return each stopper to its own bottle so reagents are never cross contaminated, heat a tube with a holder with its mouth pointed away from you, and wash your hands before leaving.
To prepare 250 cm3 of an approximately 0.1 N (N/10) solution of oxalic acid and to use it to find the normality and the strength in grams per litre of a given sodium hydroxide solution.
A 250 cm3 volumetric flask, a balance with weight box, a watch glass, a funnel, a glass rod, a wash bottle, a 50 cm3 burette with stand, a 20 cm3 pipette with filler, two conical flasks, a white glazed tile, crystalline oxalic acid of analytical grade, the given sodium hydroxide solution and phenolphthalein.
Oxalic acid crystallises as the stable dihydrate, so it neither loses nor gains water on the balance pan and can be weighed accurately, which makes it a primary standard. Its molar mass is 126 g per mole and it is dibasic, so its equivalent mass is 63 and a 0.1 N solution holds 6.3 g per litre, that is 1.575 g in 250 cm3. Sodium hydroxide is deliquescent and absorbs carbon dioxide from the air, so its solution must be standardised against the acid rather than weighed out. The neutralisation is
$$\mathrm{H_2C_2O_4 + 2NaOH \longrightarrow Na_2C_2O_4 + 2H_2O}$$
and at the end point $N_1 V_1 = N_2 V_2$. Sodium oxalate is the salt of a weak acid and a strong base, so the equivalence point is faintly alkaline and phenolphthalein, changing over roughly pH 8.3 to 10, is the correct indicator; methyl orange would change while acid was still in excess.
Rule up five columns before you begin: the number of the observation, the volume of alkali taken in cm3, the initial burette reading in cm3, the final burette reading in cm3, and the volume of acid used in cm3, which is the difference of the two readings. Allow four rows and mark the first titration as the rough one, which only shows roughly where the end point lies. Readings are concordant when two or, better, three successive titres agree within 0.1 cm3; average those alone. Record the end point change as pink to colourless.
The mean titre in $N_1 V_1 = N_2 V_2$ gives the normality of the alkali, and multiplying by 40, the equivalent mass of sodium hydroxide, gives its strength in grams per litre. State both in a closing sentence.
Weigh the acid on a watch glass and not on the pan, and make up to the mark with the flask on the bench, reading the lower meniscus with the eye level to avoid parallax. Rinse the burette with the acid and the pipette with the alkali, since water inside would dilute them, and never blow out the last drop from the pipette. Remove the funnel before the initial reading and expel the air bubble. Use only two drops of indicator, phenolphthalein being itself a weak acid, and read as soon as the colour changes, since the pink slowly returns as the flask absorbs carbon dioxide.
To prepare 250 cm3 of an approximately 0.1 N (N/10) solution of anhydrous sodium carbonate and to use it to find the normality and the strength of a given hydrochloric acid solution.
A 250 cm3 volumetric flask, a balance with weight box, a watch glass, a funnel, a glass rod, a wash bottle, a 50 cm3 burette with stand, a 20 cm3 pipette with filler, two conical flasks, a white tile, pure anhydrous sodium carbonate, the given hydrochloric acid and methyl orange.
Anhydrous sodium carbonate can be dried and weighed accurately, so it is a primary standard for acids. Its molar mass is 106 g per mole and it acts as a dibasic base here, so its equivalent mass is 53 and a 0.1 N solution holds 5.3 g per litre, that is 1.325 g in 250 cm3. Hydrochloric acid is volatile and its bottle strength cannot be trusted, so it is standardised against the carbonate:
$$\mathrm{Na_2CO_3 + 2HCl \longrightarrow 2NaCl + H_2O + CO_2}$$
The solution at the end point holds dissolved carbon dioxide, which is weakly acidic, so the equivalence point lies on the acid side of neutral and methyl orange, changing over roughly pH 3.1 to 4.4, is used. Phenolphthalein would change at the earlier stage, when the carbonate has been converted only as far as bicarbonate, and would give about half the titre.
Before you revise from this chapter, open your own school practical record book and check the exact list of experiments and the mark weighting your laboratory has been set, because the prescribed practical list varies slightly from school to school and is sometimes trimmed or reordered by the lab in charge. What follows is the practical work that is standard in Grade 11 chemistry laboratories in Nepal and that examiners routinely set: volumetric analysis by titration, qualitative analysis of acid and basic radicals, correct handling of volumetric glassware, and the determination of a physical constant such as a melting point.
The practical examination is held in your own laboratory, normally by an internal examiner working with an external examiner. You work alone at a bench under supervision, and the marks are divided between the write up you produce on the day, the accuracy of your result, your handling of apparatus, the record book you submit, and a short oral viva. Neatness and honesty carry real marks. Record every reading in ink at the moment you take it, in a table ruled up before you begin, and never nudge a reading to make the arithmetic tidy: titres that converge honestly to within 0.1 cm3 earn full credit, while identical readings in every trial only make an examiner suspicious.
Laboratory discipline is part of the assessment. Tie long hair back and wear an apron. Never draw liquid into a pipette by mouth; use a filler, which matters most with acids and with lead and barium salts, all of which are poisonous. When diluting, add concentrated acid to water and never water to concentrated acid, and smell a gas by wafting it towards you with your hand. Handle hydrogen sulphide, concentrated ammonia and concentrated acids in a fume cupboard or beside an open window. Keep the bench dry, label every test tube, return each stopper to its own bottle so reagents are never cross contaminated, heat a tube with a holder with its mouth pointed away from you, and wash your hands before leaving.
To prepare 250 cm3 of an approximately 0.1 N (N/10) solution of oxalic acid and to use it to find the normality and the strength in grams per litre of a given sodium hydroxide solution.
A 250 cm3 volumetric flask, a balance with weight box, a watch glass, a funnel, a glass rod, a wash bottle, a 50 cm3 burette with stand, a 20 cm3 pipette with filler, two conical flasks, a white glazed tile, crystalline oxalic acid of analytical grade, the given sodium hydroxide solution and phenolphthalein.
Oxalic acid crystallises as the stable dihydrate, so it neither loses nor gains water on the balance pan and can be weighed accurately, which makes it a primary standard. Its molar mass is 126 g per mole and it is dibasic, so its equivalent mass is 63 and a 0.1 N solution holds 6.3 g per litre, that is 1.575 g in 250 cm3. Sodium hydroxide is deliquescent and absorbs carbon dioxide from the air, so its solution must be standardised against the acid rather than weighed out. The neutralisation is
and at the end point . Sodium oxalate is the salt of a weak acid and a strong base, so the equivalence point is faintly alkaline and phenolphthalein, changing over roughly pH 8.3 to 10, is the correct indicator; methyl orange would change while acid was still in excess.
Rule up five columns before you begin: the number of the observation, the volume of alkali taken in cm3, the initial burette reading in cm3, the final burette reading in cm3, and the volume of acid used in cm3, which is the difference of the two readings. Allow four rows and mark the first titration as the rough one, which only shows roughly where the end point lies. Readings are concordant when two or, better, three successive titres agree within 0.1 cm3; average those alone. Record the end point change as pink to colourless.
The mean titre in gives the normality of the alkali, and multiplying by 40, the equivalent mass of sodium hydroxide, gives its strength in grams per litre. State both in a closing sentence.
Weigh the acid on a watch glass and not on the pan, and make up to the mark with the flask on the bench, reading the lower meniscus with the eye level to avoid parallax. Rinse the burette with the acid and the pipette with the alkali, since water inside would dilute them, and never blow out the last drop from the pipette. Remove the funnel before the initial reading and expel the air bubble. Use only two drops of indicator, phenolphthalein being itself a weak acid, and read as soon as the colour changes, since the pink slowly returns as the flask absorbs carbon dioxide.
To prepare 250 cm3 of an approximately 0.1 N (N/10) solution of anhydrous sodium carbonate and to use it to find the normality and the strength of a given hydrochloric acid solution.
A 250 cm3 volumetric flask, a balance with weight box, a watch glass, a funnel, a glass rod, a wash bottle, a 50 cm3 burette with stand, a 20 cm3 pipette with filler, two conical flasks, a white tile, pure anhydrous sodium carbonate, the given hydrochloric acid and methyl orange.
Anhydrous sodium carbonate can be dried and weighed accurately, so it is a primary standard for acids. Its molar mass is 106 g per mole and it acts as a dibasic base here, so its equivalent mass is 53 and a 0.1 N solution holds 5.3 g per litre, that is 1.325 g in 250 cm3. Hydrochloric acid is volatile and its bottle strength cannot be trusted, so it is standardised against the carbonate:
The solution at the end point holds dissolved carbon dioxide, which is weakly acidic, so the equivalence point lies on the acid side of neutral and methyl orange, changing over roughly pH 3.1 to 4.4, is used. Phenolphthalein would change at the earlier stage, when the carbonate has been converted only as far as bicarbonate, and would give about half the titre.