### Chemistry Past Questions

4006

A solution containing 0.095 mol. dm$$^{-3}$$ of trioxonitrate (V) acid. Solution B contains 13.50g of X$$_2$$CO$$_3$$.10H$$_2$$O per dm$$^3$$

(a) Put A in the burette and titrate with 20cm$$^3$$ or 25cm$$^3$$ portions of B using methyl orange as an indicator. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume of A used.

(b) From your results and the information provided, calculate the;

(i) Concentration of B in mol. dm$$^3$$

(ii) molar mass of X$$_2$$CO$$_3$$.10H$$_2$$O

(iii) percentage by mass of X in X$$_2$$CO$$_3$$.10H$$_3$$O. The equation for the reaction is X$$_2$$CO$$_3$$ + 2HNO$$_3{(aq)}$$ $$\to$$ 2XNO$$_{3(aq)}$$ + CO$$_{2(g)}$$ + 11H$$_2$$O$$_{(l)}$$ [H = 1, C = 12, O = 16]

(c) Give the reason for the following:
(i) using just a small quantity of the indicator during acid-base titrations.

(ii) obtaining at least two sets of readings for titration experiment.

WAEC 1993
4007

(a)(i) Define a standard solution

(ii) Give the reason why a standard solution of sodium hydroxide cannot be made by weighing out accurately a given mass of the solid and making it up to the required volume of solution

(b) Give two ways by which a solid solute can be made to dissolve more quickly in a liquid Solvent

(i) Draw a labelled diagram of the apparatus used for drying solids in the laboratory

(c) State what would be observed if a fresh precipitate of silver chloride was;

(i) Exposed to light.

(ii) Shaken will excess aqueous ammonia

WAEC 1992
4008

Credit will be given for strict adherence to instructions, for observations precisely recorded and for accurate inferences. All tests observations and inferences must be clearly entered in your answer book. in ink, at the time they are made.

H is a mixture of an element and an organic compound. Carry out the following exercises on H. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test

(a) Put all of H in a beaker and add about 10cm$$^3$$ of distilled water. Stir the mixture thoroughly and filter  Keep both the filtrate and the residue. Test the filtrate with litmus paper.

(b) Divide the filtrate into two portions

(i) To the first portion add two to three drops of acidified potassium tetraoxomanganate (VII) and warm

(ii) To the second portion. add a few drops of a saturated solution of sodium hydrogentrioxocarbonate (IV)

(c)(i) Put all the residue in a test tube and add 5cm$$^3$$ of dilute hydrochloric acid

(ii) To the resulting solution from (C)(i) above add aqueous sodium hydroxide in drops until it is in excess

(iii) From your inferences in (c)(i) and (c)(i) state what would be observed if 5cm$$^3$$ of dilute trioxonitrate (V) acid were added to a portion of the residue and the mixture was warmed.

WAEC 1992
4009

All your burette readings (initial and final), as well as the size of your pipette, must be recorded but on no account of experimental procedure is required. All calculations must be done in your answer book.

F is a solution, O a dibasic acid H$$_2$$X. G is a solution containing 1.00g of sodium hydroxide in 250cm$$^3$$ of solution.

(a) Put F in the burette and titrate with 20cm$$^3$$ or 25cm$$^3$$ portion of G using methyl orange as indicator. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume of F used

(b) From your results and the information provided, calculate the;

(i) concentration of G in mol. dm$$^3$$

(ii) concentration of F in mol. dm$$^{-3}$$

(iii) molar mass of the acid H$$_2$$X, given that 100cm$$^3$$ of solution F contained 0.4850 the acid.

The equation for the reaction is H$$_2$$X$$_{(aq)}$$ + 2NaOH$$_{(aq)}$$ $$\to$$ Na$$_2$$X$$_{(aq)}$$ + 2H$$_2$$O [H = 1; O = 16; Na = 23]

WAEC 1992
4010

(a) Name the apparatus used in the laboratory to (i) Convert vapour into liquid during distillation

(ii) determine the volumetric composition of water

(ii) produce an intermittent supply of any gas which can be evolved by the action of a liquid on a solid without heating.

(b) Give the reason for the following;

(i) the edge of the lid of a desiccator should be greased

(ii) a standard solution of sodium hydroxide is not prepared by weighing out accurately a given mass of the solid and making it up to the required volume.

iii) after the reduction of copper (II) oxide by a a stream of hydrogen gas passed overs the heated oxide in a combustion tube, it is necessary to continue passing the gas over the residue until the residue cools.

(c) An acid-base indicator turns orange in an acidic solution, green in a neutral solution and blue in an alkaline solution. The table shows the results obtained when a few drops of the indicator were added to samples of some liquids.

 Liquid Colour of the indicator in the liquid Glucose solution  Rainwater  Lime water  Vinegar Stomach powder solution Green Pale orange  Blue  Orange  Pale blue

(i) Arrange the liquids in increasing order of their expected pH values.

(ii) Explain why the colour of the indicator in the sample of rainwater is pale orange whereas in pure water it is green.

(iii) A given crystalline solid is suspected to be either sodium chloride or ammonium tetraoxosulphate (VI). Describe how you would use the indicator to identify the solid.

(iv) State with reason whether or not phenolphthalein indicator can be used to distinguish between the samples of glucose solution and vinegar.

(v) What would be the colour of methyl orange indicator in the lime water sample?

WAEC 1991