2025 NECO GCE CHEMISTRY: 2025 NECO GCE CHEMISTRY PRACTICAL ANSWERS (7690)

(1ai)
Methyl orange

(1aii)
The titration is between strong acid and weak base

(1aiii)
(i) volumetric flask
(ii) conical flask

(1aiv)
I - Yellow
II - Pink.

(1bi)
500cm³ contains 0.1mole
:. 1000cm³ will contain
= 0.1/1000/500
= 0.2mol/dm³

(1bii)
Molar mass of HCl = 1+35.5 = 36.5g/mol
Concentration of A in mol/dm³
= Conc. Of A in g/moldm-³/Molar mass of A
0.2 = Conc. Of A in g/moldm-³/36.5
Conc. of A in g/dm³ = 0.2*36.5
= 7.3g/dm³

(1biii)
CAVA/CBVB = nA/nB
CB = CAVAnB/VBnA
CB = 0.2*24*5*1/25*2
CB = 0.098mol/dm³
:. Concentration of B in moldm-³ = 0.098mol/dm³

(1biv)
Concentration of B in g/dm³
250cm³ contains 7.15g of Na2CO3.nH20
:. 1000cm³ will contain
= 7.15*1000/250 = 28.6g/dm³

(1bv)
Volume of N in Na2CO3.nH20
Conc. of B in mol/dm³ = Conc. of B in g/dm³/Molar mass
0.098 = 28.6/Molar mass
Molar mass = 28.6/0.098
= 291.84g/mol
Na2CO3.nH20 = 291.84
(23*3)+12+(16*3)+n((1*2)+16) = 291.84
46+12+8+18n = 291.84
106+18n = 291.84
18n = 291.84 - 106
18n = 185.84
n = 185.84/18 = 10.3
n = 10

(1bvi)
Percentage of water of crystallization = 10H2O/291.84 * 100/1
= 10(18)/291.84 = 180/291.84
= 61.7%

(2ai)
OBSERVATIONS;
C dissolved in distilled water to form solution

(2aii)
INFERENCE
Ca²+, Pb²+, Al³+ or Pb²+ present

(2aiii)
INFERENCE
Ca²+ present

(2aiv)
INFERENCE
Ca ²+ present

(2av)
OBSERVATIONS
a white precipitate is formed

(2bi)
INFERENCE
Co² gas from Co²,³ ^- or HCO^-³ confirmed

(2bii)
INFERENCE
Co² gas confirmed

(2biii)
INFERENCE
Ca O present

(3ai)
(i) Soda like is used because it does not attack glass
(ii) The delivery tube is withdrawn immediately heating is stopped to prevent the sucking back of water into the flask

(3aii)
(i) This is due to the formation of metallic silver which changes the colour to grey
(ii) A white fumes are formed

(3b)
(i) Retort stand
(ii) Separating funnel
(iii) Separating funnel method