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Chemistry paper 3 how to answer questions

HOW TO TACLE CHEMISTRY PRACTICAL QUESTIONS            qualitative analysis

You are provided with solid N. Carry out the test below on N. Record your observations and

inferences in the spaces provided below.

a)         Put a spatula end-ful of solid N in a boiling tube. Add about 10cm3 of distilled

water and shake thoroughly. Filter to obtain the filtrate and the residue.

Observations Inferences
      (1mk)  (1mk)

            b)         Divide the filtrate into five different portions. To the first portion add  2M NaOH

                        solution drop wise until in excess

Observations Inferences
  (1mk)    (2mk)

            c)         To the second portion add 4 drops of dilute hydrochloric acid solution

Observations Inferences
   (1mk)    (1 ½ mk)

            d)         To the third portion add 2M NH3 solution dropwise until excess

Observations Inferences
   (1mk)    (½mk)

            e)         To the fourth portion add 3 drops of lead II nitrate and heat the mixture.

Observations Inferences
                                                        (1mk)    (1mk)

            f)         Transfer all the residue into a test-tube. Add about 5cm3 of dilute nitric r acid.

                        Test for the gases using litmus paper.

Observations Inferences
   (1mk)    (½mk)

            g)         To small amount of the filtrate above add 2M NH3(aq) dropwise until in excess.

Observations Inferences
 (1mk)    (½mk)

3.         You are provided with solid T. Carry out the tests below and record your observations and

inferences.

            a)         Using a metallic spatula, burn half of solid T on a non-luminous flame of a Bunsen

                        burner,

Observations Inferences
   ( ½ mk)    (1mk)

            b)         Put the remaining amount of solid T in a boiling tube. Add about 10cm3 of distilled

water and shake thoroughly. Divide the mixture into two portions. To the first portion

add about 0.3g of sodium hydrogen carbonate

Observations Inferences
 ( ½ mk)    (½ mk)

            c)         To the second portion, add 3 drops of acidified KMnO4(aq)

Observations Inferences
   ( ½ mk)    (1mk)

ANSWERS

2.a)     

Colourless filtrate ü White residue Soluble salt and insoluble substance ü

b)

White ü ½ precipitate soluble in excess ü ½ Al3+, Zn2+, Pb2+ or Ba2+ ü ½ each

c)

No white precipitate ü ½ No efferrescence ü ½ Pb2+ absent SO32- or CO32- absent ü ½ each

d)

White precipitate insoluble ü ½ in excess Al3+ present ü ½

e)        

White precipitate ü ½ insoluble on heating ü ½ SO42- present ü

f)        

Efferrescence producing ü ½ colourless gas which change blue litmus to red ü ½ CO32- Present ü ½

g)

White precipitate ü ½ Insoluble in excess ü ½ Pb2+ present ü ½

3.         a)

White solid melts as ü ½ it burns with a yellow sooty flame   Accept a long chain hydrocarbon // insalivated organic cpd

            b)        

Efferescence / Gas bubbles ü ½ H+ // -COOH Present ü ½ .

            c)        

Purple acidified KMnO4 is decolourised ü ½   Penalise fully for any contradiction eg RoH present

You are provided with solid N. carry out the tests below, write your observations and inferences in the spaces provided.

Test Observation Inferences
(a) Take a spatula endful of N in a test-tube and add distilled water until half-filled. Shake well and divide the solution into 5 portions     (1mk)     (1mk)
(b) To the first portion add 2M NaOH solution drop wise until in excess   (2mks)  (1mk)
(c)To the 2nd portion add 2M NH3(aq) drop wise until in excess  (2mks)  (1mk)
(d) To the 3rd portion add 3drops of 2M HCl solution  (1mk)  (1mk)
(e) To the 4th portion add about 1cm3 of 2M Pb(NO3)2 solution  (1mk)  (2mks)
(f) To the 5th portion add about 1cm3 of 2M Ba(NO3)2 solution followed by dilute nitric acid.    (2mks)    (1mk)
Observation Inferences
Dissolves √ ½ to form √ ½ a colourless solution (1mk) Soluble salt present√ ½ coloured salt IONS absent (1mk)
White ppt. √ 1mk soluble in excess √ 1mk   (2mks) Pb2+, Zn2+ , Al3+ present 3ions -1mk 2ions – ½ mk 1 ion – omk Penalize ½ mk for any contradicting ion  (1mk)
(c) white ppt √ 1mk Insoluble in excess √ 1mk(2mks) Pb2+, Al3+ present √ 1mk  
(d) No White ppt √  (1mk) Al3+ present √ 1mk
(e) white ppt√ (1mk) SO32-  , SO42- , CO3 4 ions – 2mks3ions -1mk 2ions – ½ mk 1 ion – omk Penalize ½ mk for any Contradicting ion (1mk)
(f) white ppt. √ 1mk  Insoluble in dilute nitric acid √ 1mk(2mks) SO42- Present Award 0mk if ion  not mentioned in part (e) (1mk)

PART II

Titration

  • Complete table – 1mk marked as below
  • Complete table -1mk
  • Incomplete table with 2 consistent titre values – ½ mk
  • Incomplete table with inconsistent titre values =0mk

Penalties- To be penalized once

  • Inverted table
  • Arithmetic error
  • Unrealistic titre readings (unless explained)

Decimal point -1mk

  • Accept 1 d.p or 2d.p used consistently
  • If to 2 d.p the last digit to be zero or five

Accuracy -1mk

  • Any of the titre values compared to school average value
  • If within ±0.1cm3 -1mk
  • If within ±0.02cm3 – √ ½
  • If outside ±0.2cm3 -0mk