2019 NECO Chemistry Practical Answer & Question

CHEMISTRY PRACTICAL:

(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
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.

+++++++++++++++++++++++++++
(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.

COMPLETED…

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