C is an organic compound. Carry out the following exercises on C. Record your observations and identity any gas(es) evolved. State the conclusions you draw from the results of each test.
(a) Put about 10 drops of C on a watch glass and ignite it using a burning splint.
(b)(i) Put about 1 cm\(^3\) of C in a test tube and add about 1 cm\(^3\) of distilled water. Shake the test tube.
(ii) Put about 1 cm\(^3\) of C in a test tube and add about 2 cm\(^3\) of acidified K\(_2\)Cr\(_2\)O\(_7\) solution. Warm the mixture gently and leave to stand for 5 minutes.
(c) Put few crystals of specimen D in a test tube and add about 2cm\(^3\) of C followed by about 2 cm\(^3\) of 10 % NaOH\(_{(aq)}\) Shake the test tube vigorously.
(d) State the class of compounds to which C belongs.
Results of tests on C
Test
Observation
Conclusion / inference
(a) C on a watch glass, ignited with a burning splint
C burns readily with a clean, non-sooty blue flame. No solid residue remains.
C is a flammable saturated organic compound. The gaseous products of complete combustion are carbon(IV) oxide, CO2, and water vapour, H2O.
(b)(i) C + distilled water; shake
One clear homogeneous layer is formed.
C is miscible with water.
(b)(ii) C + acidified K2Cr2O7(aq); warm gently and allow to stand
The orange dichromate(VI) solution changes to green.
C is oxidised while dichromate(VI) ions are reduced to Cr3+. C is a primary or secondary alkanol.
(a) A zinc salt, E when heated strongly, produced a brown gas with pungent smell, a colourless gas that rekindled a glowing splint, and a residue that was allowed to cool.
(i) identify the salt E.
(ii) Write an equation for the decomposition of E.
(iii) State what would be observed when the residue was allowed to cool.
(b) Describe how 250cm\(^3\) of 0.2 mol dm\(^3\) H\(_2\)SO\(_4\) could be prepared from 150 cm\(^3\) of a 1.0 mol dm\(^{3}\) stock solution of the acid.
(c) State the effect of aqueous solution of Al\(_2(SO_4)_3\) on litmus paper.
(a) Identifying the zinc salt E
On strong heating E gives a brown pungent gas (nitrogen(IV) oxide, \(NO_2\)), a colourless gas that rekindles a glowing splint (oxygen, \(O_2\)) and a solid residue. These are the products of thermal decomposition of a zinc nitrate.
(iii) The residue is zinc oxide, which is yellow while hot but turns white on cooling.
(b) Preparing 250 cm\(^3\) of 0.2 mol dm\(^{-3}\) H2SO4 from a 1.0 mol dm\(^{-3}\) stock
Moles of acid required \(= 0.2 \times \dfrac{250}{1000} = 0.05\,mol\).
Volume of stock needed \(= \dfrac{0.05}{1.0} = 0.05\,dm^3 = 50\,cm^3\).
Procedure: using a pipette or measuring cylinder, measure 50 cm\(^3\) of the 1.0 mol dm\(^{-3}\) stock acid and add it slowly, with stirring, to some distilled water already in a 250 cm\(^3\) volumetric flask (always add acid to water). Then make up the solution to the 250 cm\(^3\) mark with more distilled water, stopper and shake to mix thoroughly. (The 150 cm\(^3\) of stock available is more than the 50 cm\(^3\) required.)
(c) Effect of aluminium tetraoxosulphate(VI) solution on litmus
Aqueous \(Al_2(SO_4)_3\) turns blue litmus red, i.e. the solution is acidic. This is because the salt of a strong acid and a weak base undergoes hydrolysis to give an acidic solution.
On strong heating E gives a brown pungent gas (nitrogen(IV) oxide, \(NO_2\)), a colourless gas that rekindles a glowing splint (oxygen, \(O_2\)) and a solid residue. These are the products of thermal decomposition of a zinc nitrate.
(iii) The residue is zinc oxide, which is yellow while hot but turns white on cooling.
(b) Preparing 250 cm\(^3\) of 0.2 mol dm\(^{-3}\) H2SO4 from a 1.0 mol dm\(^{-3}\) stock
Moles of acid required \(= 0.2 \times \dfrac{250}{1000} = 0.05\,mol\).
Volume of stock needed \(= \dfrac{0.05}{1.0} = 0.05\,dm^3 = 50\,cm^3\).
Procedure: using a pipette or measuring cylinder, measure 50 cm\(^3\) of the 1.0 mol dm\(^{-3}\) stock acid and add it slowly, with stirring, to some distilled water already in a 250 cm\(^3\) volumetric flask (always add acid to water). Then make up the solution to the 250 cm\(^3\) mark with more distilled water, stopper and shake to mix thoroughly. (The 150 cm\(^3\) of stock available is more than the 50 cm\(^3\) required.)
(c) Effect of aluminium tetraoxosulphate(VI) solution on litmus
Aqueous \(Al_2(SO_4)_3\) turns blue litmus red, i.e. the solution is acidic. This is because the salt of a strong acid and a weak base undergoes hydrolysis to give an acidic solution.
All your burette readings (initial and final), as well as the size of your pipette, must be recorded but no account of the experimental procedure is required. All calculations must be done in your answer booklet.
A is a solution containing 5.00 g of \( \mathrm{HNO}_3 \) in 500 cm\(^3\) of solution. B is a solution of NaOH of unknown concentration.
(a) Put A into the burette and titrate it with 20.0 cm\(^3\) or 25.0 cm\(^3\) portions of B using methyl orange as an indicator. Repeat the titration to obtain concordant titre values. Tabulate your results and calculate the average volume of acid used. Equation of the reaction is \( \mathrm{HNO}_{3(aq)} + \mathrm{NaOH}_{(aq)} \to \mathrm{NaNO}_{3(aq)} + \mathrm{H}_2\mathrm{O}_{(l)} \)
(b) From your results and the information provided. calculate the: (i) concentration ot A In mol dm\(^{-3}\)
(ii) concentration of B in mol dm\(^{-3}\).
(iii) concentration of B in gdm\(^{-3}\)
(iv) mass of NaNO\(_3\) formed. If 250 cm\(^3\) of NaOH were neutralised. [Molar mass of NaOH = 40g mol\(^{-1}\), NaNO\(_3\) = 85 gmol\(^{-1}\). Credit will be given for strict adherence to the instructions. for observations precisely recorded and for accurate inferences. All tests, observations and inferences must be clearly entered in this booklet, in ink, at the time they are made.
