properties-and-masses-of-compounds MCQs for UPSC Prelims

25 practice questions on properties-and-masses-of-compounds from the Atomic Foundations of Matter section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

8 Easy 12 Medium 5 Hard

Sample questions

Q1
hard
A molecule of glucose, the sugar carried in our blood, contains six carbon atoms, twelve hydrogen atoms and six oxygen atoms. Atomic masses are $\text{C} = 12\ \text{u}$, $\text{H} = 1\ \text{u}$ and $\text{O} = 16\ \text{u}$. What is its molecular mass?
  1. A $180\ \text{u}$
  2. B $29\ \text{u}$
  3. C $168\ \text{u}$
  4. D $174\ \text{u}$
Show answer and explanation

Correct answer: A - $180\ \text{u}$

Six carbon atoms give $12 \times 6 = 72\ \text{u}$, twelve hydrogen atoms give $1 \times 12 = 12\ \text{u}$, and six oxygen atoms give $16 \times 6 = 96\ \text{u}$, so the total is $72 + 12 + 96 = 180\ \text{u}$. Adding just one atom of each gives $29\ \text{u}$, leaving the hydrogen out gives $168\ \text{u}$, and counting only six hydrogen atoms gives $174\ \text{u}$.
Q2
medium
Atomic masses are $\text{Mg} = 24\ \text{u}$, $\text{O} = 16\ \text{u}$ and $\text{H} = 1\ \text{u}$. What is the formula unit mass of magnesium hydroxide, $\text{Mg}(\text{OH})_2$?
  1. A $82\ \text{u}$
  2. B $41\ \text{u}$
  3. C $57\ \text{u}$
  4. D $58\ \text{u}$
Show answer and explanation

Correct answer: D - $58\ \text{u}$

The subscript outside the bracket applies to the whole hydroxide group, so two $\text{OH}$ groups give $(16 + 1) \times 2 = 34\ \text{u}$, and with magnesium the total is $24 + 34 = 58\ \text{u}$. Using only one hydroxide group gives $41\ \text{u}$. Doubling the oxygen but not the hydrogen gives $57\ \text{u}$, and doubling the entire formula gives $82\ \text{u}$.
Q3
medium
Atomic masses are $\text{N} = 14\ \text{u}$ and $\text{H} = 1\ \text{u}$. What is the molecular mass of ammonia, $\text{NH}_3$?
  1. A $15\ \text{u}$
  2. B $17\ \text{u}$
  3. C $42\ \text{u}$
  4. D $45\ \text{u}$
Show answer and explanation

Correct answer: B - $17\ \text{u}$

One nitrogen atom gives $14\ \text{u}$ and three hydrogen atoms give $1 \times 3 = 3\ \text{u}$, so the molecular mass is $14 + 3 = 17\ \text{u}$. Ignoring the subscript gives $15\ \text{u}$, multiplying the whole formula by three gives $45\ \text{u}$, and $42\ \text{u}$ multiplies the nitrogen by three instead of the hydrogen.
Q4
medium
Sugar dissolves easily in water, yet a sugar solution does not make a bulb in a test circuit glow. What does this show?
  1. A Every solution in water conducts, so the circuit must be faulty
  2. B Sugar breaks down into water and carbon as it dissolves
  3. C Sugar is covalent and dissolves as neutral molecules, so no ions are released
  4. D Sugar is ionic, but its ions are too large to move through the water
Show answer and explanation

Correct answer: C - Sugar is covalent and dissolves as neutral molecules, so no ions are released

Sugar is a covalent compound, so it dissolves as whole neutral molecules and releases no charged particles to carry a current. It is not ionic, so the size of ions does not come into it. Dissolving does not break sugar into water and carbon, and a solution conducts only when it contains ions, so a perfectly good circuit can still leave the bulb dark.
Q5
easy
What is typical of the melting and boiling points of ionic compounds?
  1. A They are high, because strong forces hold the oppositely charged ions together
  2. B They are low, because the ions are held only weakly
  3. C They are high, because ionic compounds are always built from heavy atoms
  4. D They are exactly the same as those of covalent compounds
Show answer and explanation

Correct answer: A - They are high, because strong forces hold the oppositely charged ions together

Strong attraction between the ions holds an ionic crystal together, so a great deal of energy is needed to pull it apart and the melting and boiling points come out high. Weak forces would give low melting points, which is the covalent pattern instead. The mass of the atoms is not the reason, and covalent compounds usually melt at much lower temperatures.
Q6
easy
Atomic masses are $\text{K} = 39\ \text{u}$ and $\text{Cl} = 35.5\ \text{u}$. What is the formula unit mass of potassium chloride, $\text{KCl}$?
  1. A $39\ \text{u}$
  2. B $74.5\ \text{u}$
  3. C $149\ \text{u}$
  4. D $3.5\ \text{u}$
Show answer and explanation

Correct answer: B - $74.5\ \text{u}$

A formula unit of $\text{KCl}$ holds one potassium atom and one chlorine atom, so its mass is $39 + 35.5 = 74.5\ \text{u}$. Subtracting instead of adding gives $3.5\ \text{u}$, doubling the sum gives $149\ \text{u}$, and $39\ \text{u}$ leaves the chlorine out altogether.
Q7
easy
Camphor dissolves readily in kerosene but hardly at all in water. What does this suggest about camphor?
  1. A It is a metal
  2. B It releases free ions when it dissolves
  3. C It is a covalent compound
  4. D It is an ionic compound
Show answer and explanation

Correct answer: C - It is a covalent compound

Dissolving in kerosene rather than in water is typical covalent behaviour, and camphor is indeed a covalent compound. Ionic compounds show the opposite preference, dissolving in water and not in kerosene. Camphor is a soft white solid rather than a metal, and a covalent compound releases no free ions when it dissolves.
Q8
hard
Atomic masses are $\text{H} = 1\ \text{u}$, $\text{P} = 31\ \text{u}$ and $\text{O} = 16\ \text{u}$. What is the molecular mass of phosphoric acid, $\text{H}_3\text{PO}_4$?
  1. A $158\ \text{u}$
  2. B $48\ \text{u}$
  3. C $95\ \text{u}$
  4. D $98\ \text{u}$
Show answer and explanation

Correct answer: D - $98\ \text{u}$

Three hydrogen atoms give $1 \times 3 = 3\ \text{u}$, one phosphorus atom gives $31\ \text{u}$, and four oxygen atoms give $16 \times 4 = 64\ \text{u}$, so the total is $3 + 31 + 64 = 98\ \text{u}$. Ignoring both subscripts gives $48\ \text{u}$, leaving the hydrogen out gives $95\ \text{u}$, and attaching the subscript $3$ to phosphorus instead of hydrogen gives $158\ \text{u}$.

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