Chemistry

Mole Concept and Stoichiometry

354 Questions

The mole concept is a core area of chemistry focusing on the quantitative relationships between reactants and products in chemical reactions. Questions cover molecular mass calculations, stoichiometric conversions, and determining the number of molecules. This topic is essential for most science entrance and competitive examinations.

molecular mass calculationsstoichiometric conversionsmole fraction problemsheavy water reactionsoxidation calculations

Mole Concept and Stoichiometry Questions

Multiple choice

The molecular formula of heavy water is-

  1. D3O

  2. D2O

  3. DO

  4. DO2
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Heavy water or deuterium oxide has formula D2O, where D is deuterium (hydrogen-2 isotope with one neutron). D3O is incorrect stoichiometry, DO doesn't exist as stable molecule, and DO2 is not a valid formula.

Multiple choice

A traditional chemist can synthesize _____________ compounds per year.

  1. 100-200

  2. 200-300

  3. 300-400

  4. 400-500
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Traditional synthetic chemists working manually can typically synthesize 100-200 new compounds per year. This estimate reflects the time-intensive nature of purification, characterization, and optimization required for each novel compound in classical synthetic workflows.

Multiple choice

Which of the following statements is false about a compound, which is combined in the fixed proportion?

  1. 4 grams of hydrogen combine with 32 grams of oxygen to give 36 grams of water.

  2. In the molecule of carbon dioxide, carbon and oxygen are present in fixed ratio of 3 : 8 by mass.

  3. In the molecule of water, hydrogen and oxygen are present in the ratio of 1 : 8.

  4. 12 grams of carbon combine with 32 grams of oxygen to give 40 grams of carbon dioxide.

  5. In the molecule of carbon monoxide (CO), the ratio of carbon and oxygen is 3 : 8.
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

In the molecule of carbon monoxide (CO), the ratio of carbon and oxygen is 3 : 4,  but not 3 : 8 [CARBON : OXYGEN = 12 : 16   = 3 : 4]

Multiple choice

A compound is made up of elements X and Y. The equivalent weight of X is one-fourth its atomic weight and the equivalent weight of Y is half its atomic weight. Therefore, the formula of the compound is

  1. XY

  2. XY2

  3. X2Y

  4. X4Y2
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A compound is made of elements X and Y. The equivalent weight of X is one-fourth its atomic weight and the equivalent weight of Y is half its atomic weight. Therefore, the formula of the compound is XY2.

Multiple choice

Plaster of Paris is

  1. CaSO4 .0.5 H2O

  2. CaSO4.2H2O

  3. CaSO4.7H2O

  4. CaSO4.3H2O
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Plaster of Paris is calcium sulfate hemihydrate with the formula CaSO₄·½H₂O (or written as 2CaSO₄·H₂O). When mixed with water, it forms gypsum (CaSO₄·2H₂O) and hardens. Option B is already gypsum, while C and D have incorrect water content.

Multiple choice

Calculate the total number of molecules in 4.4 g of CO2 (g).

  1. 6.022 * 10^23

  2. 6.022 * 10^22

  3. 6.022 * 10^21

  4. 6.022 * 10^20

  5. 6.022 * 10^29
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

No. of moles CO2 = (4.4 / 44) = 0.1 No. of molecules = 0.1 * 6.022 * 10^23 = 6.022 * 10^22, which match with the given option, hence this is a correct option.

Multiple choice

1.204 × 1021 molecules are removed from 392 mg of sulphuric acid. How many moles of sulphuric acid are left?

  1. 1.5 × 10-3

  2. 2.0 × 10-3

  3. 2.5 × 10-3

  4. 3.0 × 10-3

  5. 3.5 × 10-3
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Used sulphuric acid = 392 mg = 0.392 g Number of moles of sulphuric acid = 0.392/98 = 0.004 Since N = nNA                  (NA = Avogadro’s number) Hence, n = N/NA = 1.204 × 1021/ 6.023 × 1023 = 0.002 moles The number of moles left = 0.004 – 0.002 = 0.002 = 2.0 × 10-3