Nabteb chemistry practical | chemistry expo |
NABTEB 2023 CHEMISTRY PRACTICAL ANSWERS
Volume of pipette = 25.0cm³
Burette Reading (cm³) | 1st | 2nd | 3rd
Final Reading (cm³) | 25.00 | 34.00 | 46.00
Initial Reading (cm³) | 0.00 | 10.00 | 20.00
Volume of A used (cm³) | 25.00 | 24.00 | 26.00
Average Volume of A used = (1st + 2nd + 3rd)/3
VA = (25+24+26)/3
VA = 25.00cm³
CB = PA/MB
CB = 5.55/56
CB = 0.0991mol dm-³
CA = PA/MA
CA = 4.12/90
CA = 0.0458moldm-³
nB = CBVB
nB = 0.0991x(25/1000)
nB = 2.4775×10-³mol
2mol of KOH requires 1mol of H₂C₂O₄
2.4775×10-³ mol of KOH = (2.4775×10-³/2) mol of H₂C₂O₄
nA = 1.23875×10-³ mol of acid
Mass of anhydrous acid = nM
Mass of water of crystallization = 4.12 – 0.11
Percentage by mass of water of crystallization = (4.01/4.12) x 100%
C + 5cm³ of distilled water + filtrate with litmus paper
Litmus paper turns blue
The filtrate is basic
2cm³ of filtrate + 2cm³ of Fehling’s solution and warm
Brick-red precipitate formed
Reducing sugar is present
Residue + 5cm³ of dilute HCL + NaOH(aq) in drops and then excess
A green precipitate formed upon the addition of NaOH(aq)
Zinc salt (Zn²⁺) confirmed
Residue + Aqueous NH₃ in drops and excess
A white precipitate formed upon the addition of aqueous NH₃
Zinc hydroxide (Zn(OH)₂) confirmed
The organic compound belongs to the class of reducing sugars.
Passing CO₂ through lime water for a long time:
Observation: The lime water gradually turns milky or cloudy due to the formation of the white precipitate.
Soap solution is shaken with the final product:
(i) Before Boiling:
Observation: The mixture appears cloudy or milky.
(ii) After Boiling:
Observation: The cloudy or milky appearance disappears, and the mixture becomes clear.
(i) Passing CO₂ through lime water for a long time: When CO₂ is passed through lime water, a chemical reaction occurs between carbon dioxide and calcium hydroxide present in lime water. This reaction results in the formation of calcium carbonate CaCO₃ and water (H₂O). The white precipitate of calcium carbonate is insoluble and appears as a milky or cloudy substance in the lime water.
(ii) Soap solution is shaken with the final product:
(a) Before boiling:
When soap solution is shaken with the milky lime water obtained, the soap molecules interact with the calcium carbonate particles present in the lime water. The hydrophilic heads of soap surround the calcium carbonate particles, forming micelles or small clusters. This disperses the calcium carbonate particles throughout the mixture, resulting in a cloudy or milky appearance.
(b) After boiling:
When the mixture is boiled it breaks down the micelles formed by soap molecules around the calcium carbonate particles. The dispersed calcium carbonate particles are no longer surrounded by soap molecules and can no longer stay dispersed in the mixture. The dissolved soap molecules remain in the clear upper layer of the mixture, resulting in its transparency.
When concentrated HNO₃ is spilled on the skin it causes a yellow stain and when ammonia solution is added to the same spot a white color is observed.