NECO 2019 - PRACTICAL CHEMISTRY ANSWER

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*TAKE NOTE ON THIS NUMBER 1*
Pls note that each school have their titre value or end point. In our solution, we used 20.40cm3. So make sure you use ur school's titre value. Ask your chemistry teacher. Some school got 24.50, 24.20, 22.10, 22.40, etc. Make sure your average titre value tallies with that of ur school's. Anywhere you see 20.40 in my work, edit it and re-solve with ur school's value.
This is very important.
Thanks.
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ANSWERS:

(1)
The volume or pipette used 25.00cm³

In a tabular form
Titrations |rough |1st|2nd|3rd|
Final reading |21.00|41.30|20.40|40.90
Initial reading|0.00|21.00|0.00|20.40
Vol of A used |21.00|20.30|20.40|20.50

(1ai)
Average volume of A = 20.30+20.40+20.50/3
=61.2/3
=20.40cm³

(1aii)
H2X(aq) + 2NaOH(aq)-->Na2X(aq) + 2H2O(l)

(1bi)
Mass conc. of B = 1g/250cm³
= 1g/0.25dm³ = 4g/dm³
Using :
Molar conc. = mass conc./molar mass
= 4/(23+16+1) = 4/40 = 0.1mol/dm³

(1bii)
Using CAVA/CBVB = nA/nB
CA = CBVBnA/VAnB
CA = 0.1×25×1/20.4×2
Concentration of A = 0.06127mol/dm³

(1biii)
Molar mass of A = mass conc/molar conc
=4.85/0.6127
=79.15
≅ 79gmol

(1c)
(i)it might contain traces of impurities which must be removed before being weighed
(ii)it is hygroscopic which readily absorb moisture from atmosphere



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(2)
FILL IN THE TABLE:

(2i)
OBSERVATION:
Solution turns blue

INFERENCE:
Copper salt suspected.

(2ii)
OBSERVATION:
Blue gelatinous precipitate

Insoluble in excess NaOH

INFERENCE:
Cu²+ suspected

Cu²+ Confirmed

(2iii)
OBSERVATION:
Blue gelatinous precipitate

Soluble in excess NH³

INFERENCE:
Cu²+ Suspected

Cu²+ Confirmed

(2iv)
OBSERVATION:
White precipitate

INFERENCE:
SO4²- Suspected

(2v)
OBSERVATION:
Precipitate insoluble in excess

INFERENCE:
SO4²- confirmed



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(3ai)
Methyl orange

(3aii)
I. Yellow
II. Orange

(3aiii)
(i)To give a sharp change in colour at the end of the acid base reaction

(3aiv)
(i) The burette must be rinsed with the acid or allow it to drain after rinsing with distilled water
(ii)Air bubble must not be allowed in the burette

(3av)
Neutralization reaction


(3bi)
I. Hofmann voltameter.
II. Kipp's apparatus

(3bii)
Because silver nitrate is photosensitive (i.e.sensitive to light) Therefore,if the silver nitrate is left in sunlight or any bright light, it breaks down (hydrolyses) to give black/brown silver oxide and nitric acid.






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