2025 NECO GCE CHEMISTRY: 2025 NECO GCE CHEMISTRY PRACTICAL ANSWERS (7781)

(1ai)

Tabulate

Burette reading/cm3 | 1st titre | 2nd titre

Final reading/cm3 | 22.50 | 22.50

Initial reading/cm3 | 0.00 | 0.00

Volume of A used/cm3 | 22.50 | 22.50

(1aii)

This is used for the easy identification of end-point or neutrality point.

(1aiii)

To avoid error in measurement which may occur through droplet of A into the burette affecting the reading

(1bi)

Mass conc. of A = 4.015/dm3

Mass conc. of B = 5.55/dm3

Volume of A, Va = 22.5cm3

Volume of B, Vb = 25.0cm3

Molar mass of A, Hcl = 1+35.5

= 36.5g/mol

:- Conc. of A in mol/dm3 = ??

Conc. of A (mol/dm3) = conc. Of A (g/dm3) ÷ molar mass

= 4.015/36.5

Conc. of A = 0.11mol/dm3

(1bii)

Conc. of B in mol/dm3 = ??

Cb = ??

Ca = 0.11mol/dm3, Va = 22.5cm3, V6 = 25.0cm3, na = 1, nb = 1

CaVa/CbVb = na/nb

Cb = CaVanb/naVa

Cb = 0.11x22.5x1/1x25.0

Cb = 2.475/25.0

Cb = 0.099mol/dm3

(1biii)

Molar mass of B = ??

ing Conc (mol/dm3) = conc. (g/dm3) ÷ molar mass

Molar mass = conc. (g/dm3) ÷ conc. (mol/dm3)

Molar mass = 5.55/0.099

Molar mass = 56.0g/mol

(1biv)

XOH = 56

X + 16 + 1 = 56

X + 17 = 56

X = 56 - 17

X = 39

(1c)

Potassium

(1cii)

It is an analytical 'copied from e x p o c o d e . n e t free' value

(1ciii)

KOH

(1civ)

Potassium hydroxide

(2ai)

Is a soluble salt.

(2aii) 

Ca²+ suspected.

(2aiii) 

Ca2+ Present. 

(2aiv)

Ppt remains insoluble.

(2av)

White Ppt formed which Remain insoluble

(2avi)

Brick red flame observed.

(2bi)

SO₂, H₂S Or CO₂ gas suspected

(2bii) 

Lead(II) Ethanoate paper turns black

(2biii)

H₂s gas Evolved. S²- present

(3ai)

I - KMnO4

II - Bromine water

III - Litmus paper

(3aii)

(i) A rotten egg odour is perceived

(ii) A colourless gas is formed

(iii) A green coloured compound is formed

(3bi)

I:

- A precipitate is formed

- Asoluble xompound is also formed

II:

- Filtration

- Decantation

(3bii)

AgNO3 (silver trioxonitrate(v)