50. Which of these has the greatest concentration of hydrogen ions under usual conditions?
[UPSC NDA — 2018]
(A) Sodium hydroxide
(B) Milk of magnesia
(C) Lemon juice
(D) Gastric juice
Answer: (D) Gastric juice
Explanation: Gastric juice is generally more strongly acidic than lemon juice and releases more hydrogen ions. Sodium hydroxide and milk of magnesia are alkaline. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
49. Which solution typically has pH below 7?
[UPSC NDA — 2018]
(A) NaOH
(B) KCl
(C) FeCl3
(D) NaCl
Answer: (C) FeCl3
Explanation: Iron(III) chloride solution becomes acidic because hydrated Fe3+ ions hydrolyse in water, liberating hydronium. Salts of strong acid and strong base are near neutral. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.
48. Brine is a concentrated aqueous solution of which substance?
[UPSC NDA — 2018]
(A) NaCl
(B) NaOH
(C) NaHCO3
(D) Na2CO3
Answer: (A) NaCl
Explanation: Brine generally refers to saltwater rich in sodium chloride. Industrial brine electrolysis is used to obtain chlorine, hydrogen and sodium hydroxide. Halogens lie in group 17 and tend to gain electrons when reacting with metals.
47. Which statement about bleaching powder is false?
[UPSC NDA — 2018]
(A) It is principally a reducing agent
(B) It can bleach wood pulp
(C) It disinfects drinking water
(D) It bleaches textile linen
Answer: (A) It is principally a reducing agent
Explanation: Bleaching powder is used primarily because of its oxidising and disinfecting action. It is not normally described as a reducing agent. Chemical structure, bonding and surrounding conditions determine why the stated choice is correct.
46. Which pair of gases is known as synthesis gas or syngas?
[UPSC NDA — 2018]
(A) Carbon and steam
(B) CO and steam
(C) CO and hydrogen
(D) NO2 and hydrogen
Answer: (C) CO and hydrogen
Explanation: Synthesis gas consists mainly of carbon monoxide and hydrogen. It serves as feedstock for methanol synthesis and other industrial chemical reactions. The structure and bonding of water-related compounds largely determine their physical and chemical properties.
45. Which ammonium compound is not commonly used as a fertiliser?
[UPSC NDA — 2018]
(A) Ammonium nitrate
(B) Ammonium sulphide
(C) Ammonium phosphate
(D) Ammonium sulphate
Answer: (B) Ammonium sulphide
Explanation: Ammonium nitrate, phosphate and sulphate supply crop nutrients. Ammonium sulphide is not a normal agricultural fertiliser and can liberate toxic hydrogen sulphide. In aqueous chemistry, acidity depends on dissolved ionic species and the prevailing conditions.
44. What is the general molecular formula of an acyclic alkane?
[UPSC CDS — 2018]
(A) CnH2n+2
(B) CnH2n
(C) CnH2n−2
(D) CnHn
Answer: (A) CnH2n+2
Explanation: Alkanes are saturated hydrocarbons with carbon-carbon single bonds only. For open-chain compounds the hydrogen count is twice the carbon count plus two; cyclic alkanes have another formula. The structure and bonding of water-related compounds largely determine their physical and chemical properties.
43. Which pair of pollutants is identified with the formation of acid rain in this RAS question?
[RPSC RAS / RPSC AEn — 2018]
(A) Carbon dioxide and nitrogen
(B) Carbon monoxide and carbon dioxide
(C) Nitrous oxide and sulphur dioxide
(D) Ozone and carbon dioxide
Answer: (C) Nitrous oxide and sulphur dioxide
Explanation: The archived examination key selects the option mentioning sulphur dioxide and a nitrogen oxide. Chemically, sulphur dioxide and NO/NO2 are major acid-rain precursors; nitrous oxide (N2O) is not a principal cause, so the printed choice is imprecise.
42. In HSO4− + H2O → H3O+ + SO4²−, what role does water perform?
[UPSC CDS — 2017]
(A) Acid
(B) Base
(C) Salt
(D) Inert medium
Answer: (B) Base
Explanation: Water accepts a proton from hydrogen sulphate and forms hydronium. This meets the Brønsted–Lowry definition of a base, even though water can act as an acid in other reactions. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
41. Which mineral makes up most of a pearl?
[UPSC CDS — 2017]
(A) Calcium carbonate
(B) Calcium oxide
(C) Calcium nitrate
(D) Calcium sulphate
Answer: (A) Calcium carbonate
Explanation: Pearls are largely crystalline calcium carbonate held in an organic matrix. Mollusks deposit layers of nacre around an irritant or within tissue. Water solubility and hydration are often as important as composition in their practical use.
