All 21 questions from the Post-UTME Screening (Post-UTME) Chemistry 2018 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
In the reaction between sodium hydroxide and sulphuric acid solutions, what volume of 0.5 molar sodium hydroxide will exactly neutralize 10cm^3 of 1.25 molar sulphuric acid?
500cm^3 of oxygen was collected over water at 30 degrees C and 752mmHg pressure. What is the volume of dry oxygen at S.T.P? (Vapour pressure of water at 30C = 32mmHg)
A. 475cm^3
B. 415cm^3
C. 500cm^3
D. 427cm^3Correct
Explanation
Dry gas pressure = 752-32 = 720mmHg. V(STP) = (720 x 500 x 273)/(760 x 303) = approx 427cm^3.
The IUPAC name for the compound CH2=C(CH3)-C(CH3)=CH2 type structure shown is
A. 2,3-dimethylpent-2,3-diene
B. 2,3-dimethylpent-2-ene
C. 3,4-dimethylpent-2-eneCorrect
D. 3,4-dimethylheptene
Explanation
Based on the branched structure shown (methyl substituents at the 3 and 4 positions of a pentene chain), the IUPAC name is 3,4-dimethylpent-2-ene. Flagged: the drawn structure in the scan was partly unclear, so this naming is a best-effort match.
A mixture contains 20cm^3 of hydrogen, some oxygen, 15cm^3 of carbon monoxide and 10cm^3 of nitrogen at S.T.P. Which of the following gives the mole fraction of hydrogen in this mixture?
A. 0.02
B. 0.16
C. 0.20
D. 0.25Correct
Explanation
Mole fraction = volume of H2 / total volume. With the oxygen volume as printed in the source, the total works out to 80cm^3, giving 20/80 = 0.25. Flagged: the oxygen volume figure was not fully legible in the scan.
In the preparation of ammonia, which of the following is used as a drying agent?
A. Calcium chloride
B. Concentrated tetraoxosulphate(VI) acid
C. Calcium oxideCorrect
D. Sodium hydroxide
Explanation
Calcium oxide (quicklime) is used to dry ammonia gas, since ammonia (a base) would react with acidic drying agents like conc. H2SO4 or form a complex with CaCl2.
When carbon (IV) oxide is bubbled into lime water, a white precipitate is first formed, which later dissolves in excess CO2 to form a colourless solution. The white precipitate and the soluble compound formed in the reactions are respectively
A. Na2CO3 and NaHCO3
B. Na2CO3 and CaCl2
C. Ca(OH)2 and CaCO3
D. CaCO3 and Ca(HCO3)2Correct
Explanation
CO2 first forms insoluble CaCO3 with limewater; with excess CO2, this converts to soluble calcium hydrogencarbonate, Ca(HCO3)2.
Using the table above (CH4 = -74.8, O2 = 0, C = -393.5, H2O = -286), calculate the delta H for the following reaction at 25C: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g).