15 extra multiple-choice questions for p-Block Elements-II (12th Standard Chemistry, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which of the following compounds found in the earth's crust is commonly referred to as Chile saltpetre?
- A. Sodium nitrateCorrect
- B. Potassium nitrate
- C. Ammonium nitrate
- D. Calcium nitrate
Explanation. Sodium nitrate is known as Chile saltpetre, whereas potassium nitrate is referred to as Indian saltpetre. Both are primary mineral sources of nitrogen.
Q2
Which allotropic form of phosphorus is poisonous, has a characteristic garlic smell, and exhibits the phenomenon of phosphorescence?
- A. Red phosphorus
- B. Black phosphorus
- C. White phosphorusCorrect
- D. Violet phosphorus
Explanation. White phosphorus is a poisonous allotrope with a garlic-like odor that glows in the dark due to slow oxidation, a property known as phosphorescence.
Q3
In the molecular structure of phosphorous trioxide (\(P_{4}O_{6}\)), how many oxygen atoms are positioned along the edges of the tetrahedron?
- A. Four
- B. SixCorrect
- C. Eight
- D. Ten
Explanation. In \(P_{4}O_{6}\), the four phosphorus atoms are located at the corners of a tetrahedron, with six oxygen atoms located along its edges.
Q4
Which of the following is the most abundant element in the earth's crust by weight, making up approximately 46.6%?
- A. Silicon
- B. Aluminium
- C. OxygenCorrect
- D. Sulphur
Explanation. Oxygen is the most abundant element, constituting about 46.6 percent of the earth's crust by weight, primarily existing in combined forms like oxides.
Q5
Sulphur exists in multiple allotropic modifications. Which forms are designated as \(\alpha\)-sulphur and \(\beta\)-sulphur?
- A. Plastic and milk sulphur
- B. Colloidal and plastic sulphur
- C. Rhombic and monoclinic sulphurCorrect
- D. Amorphous and crystalline sulphur
Explanation. Rhombic sulphur is known as alpha-sulphur, and monoclinic sulphur is known as beta-sulphur. These are the primary crystalline allotropic forms of sulphur.
Q6
Which of the following is the primary mineral source for the commercial extraction of fluorine?
- A. Cryolite
- B. FluorsparCorrect
- C. Fluorapatite
- D. Sodium chloride
Explanation. Fluorspar, also known as fluorite, is the main ore of fluorine. Other sources like cryolite and fluorapatite are also important but less primary.
Q7
Why is hydrofluoric acid (HF) classified as a weak acid, unlike the other binary acids of halogens?
- A. Fluorine is the least electronegative
- B. It has the lowest bond dissociation enthalpy
- C. Strong hydrogen bonding and high bond enthalpyCorrect
- D. It does not form any hydrogen bonds
Explanation. HF is a weak acid due to the high strength of the H-F bond and the presence of strong hydrogen bonding, making proton release difficult.
Q8
Which Group 18 element is characterized by having the smallest atomic radius and the highest first ionisation energy?
- A. Neon
- B. Argon
- C. HeliumCorrect
- D. Xenon
Explanation. Helium has the smallest atomic size and the largest ionisation energy of all elements because its electrons are held very tightly in a stable shell.
Q9
In the medical field, which radioactive noble gas is sealed in small capsules and used to destroy malignant cancer growth?
- A. RadonCorrect
- B. Krypton
- C. Xenon
- D. Argon
Explanation. Radon gas is radioactive and is used in cancer radiotherapy. It is sealed into capsules and implanted into the body to target tumors.
Q10
When white phosphorus is heated with a strong solution of sodium hydroxide in an inert atmosphere, which gas is produced along with sodium hypophosphite?
- A. Ammonia
- B. PhosphineCorrect
- C. Nitrogen
- D. Oxygen
Explanation. White phosphorus reacts with aqueous NaOH and water to form phosphine gas (\(PH_{3}\)) and sodium hypophosphite (\(NaH_{2}PO_{2}\)).
Q11
Which xenon fluoride is prepared by the direct reaction of xenon and fluorine in a 1:20 volume ratio at 400°C?
- A. \(XeF_{2}\)
- B. \(XeF_{4}\)
- C. \(XeF_{6}\)Correct
- D. \(XeOF_{4}\)
Explanation. Xenon hexafluoride (\(XeF_{6}\)) is produced when xenon and fluorine react in a 1:20 ratio at 400 degrees Celsius in a sealed nickel vessel.
Q12
In the contact process for the industrial manufacture of sulphuric acid, what is used as the catalyst for the oxidation of \(SO_{2}\) to \(SO_{3}\)?
- A. Finely divided iron
- B. Nickel powder
- C. Vanadium pentoxideCorrect
- D. Manganese dioxide
Explanation. Vanadium pentoxide (\(V_{2}O_{5}\)) is the catalyst used to increase the rate of reaction during the conversion of sulphur dioxide to sulphur trioxide.
Q13
Which of the following describes the chemical property of ozone (\(O_{3}\)) that makes it effective in disinfecting swimming pools?
- A. It is a weak reducing agent
- B. It is a strong oxidising agentCorrect
- C. It is chemically inert
- D. It is a highly stable gas
Explanation. Ozone is a powerful oxidising agent, which allows it to react with and destroy organic contaminants and pathogens in water.
Q14
Which property is responsible for the phosphine gas (\(PH_{3}\)) being used in smoke screens during naval signaling?
- A. It is highly flammable
- B. It produces dense white fumes of \(P_{2}O_{5}\)Correct
- C. It is a colorless gas
- D. It has a characteristic smell
Explanation. When phosphine burns, it produces dense white fumes of phosphorus pentoxide, which acts as an effective smoke screen for signaling.
Q15
Which oxoacid of phosphorus is prepared by the hydrolysis of phosphorous trioxide (\(P_{4}O_{6}\)) with cold water?
- A. Hypophosphorous acid
- B. Orthophosphorous acidCorrect
- C. Orthophosphoric acid
- D. Pyrophosphoric acid
Explanation. Orthophosphorous acid (\(H_{3}PO_{3}\)) is formed when phosphorous trioxide reacts with cold water in a hydrolysis reaction.
Frequently asked questions
How many MCQs are there in p-Block Elements-II?
This chapter has 15 book-back multiple-choice questions, each with the correct answer and a step-by-step explanation.
Are these 12th Standard Chemistry MCQs free to practise online?
Yes. Every question, answer and explanation here is free, and you can also take them as a timed practice test.
Where can I find the p-Block Elements-II book-back answers?
The correct option for each question is highlighted on this page with a worked explanation, plus a quick answer-key summary at the top.
