Question 1

What is the molecular mass of CO₂ (carbon dioxide)? [Atomic masses: C = 12, O = 16]

Accepted Answer

Molecular mass of CO₂ = 12 + (16 × 2) = 12 + 32 = 44 u, calculated by summing the atomic masses of all atoms in the molecule.

Question 2

What is the atomic mass unit (amu) approximately equal to?

Accepted Answer

The atomic mass unit is defined as 1/12th of the mass of a carbon-12 atom, which serves as the standard for measuring atomic masses.

Question 3

Which of the following correctly defines a molecule?

Accepted Answer

A molecule is the smallest unit of a substance formed when two or more atoms combine chemically through bonds.

Question 4

If an element has an atomic number of 8, how many electrons does a neutral atom of this element have?

Accepted Answer

In a neutral atom, the number of electrons equals the atomic number; therefore, an atom with atomic number 8 has 8 electrons.

Question 5

Which of the following represents a molecule composed of atoms of different elements?

Accepted Answer

A compound molecule contains atoms of two or more different elements; CO₂ contains carbon and oxygen atoms.

Question 6

What is the mass number of an atom that contains 8 protons and 8 neutrons?

Accepted Answer

Mass number = Number of protons + Number of neutrons = 8 + 8 = 16.

Question 7

The atomic mass unit (amu) is defined as one-twelfth of the mass of which element?

Accepted Answer

The atomic mass unit (amu) is standardly defined as 1/12th of the mass of a carbon-12 atom.

Question 8

If an atom has an atomic number of 6 and a mass number of 14, how many neutrons does it have?

Accepted Answer

Number of neutrons = Mass number − Atomic number = 14 − 6 = 8.

Question 9

What is the molar mass of a substance if one mole contains 6.02 × 10²³ molecules with a total mass of 32 grams?

Accepted Answer

Molar mass is the mass of one mole of substance; 32 grams for 1 mole equals 32 g/mol.

Question 10

The relative atomic mass of an element is the average mass of its atoms compared to 1/12th of the mass of a carbon-12 atom. If an element has a relative atomic mass of 32, how many times heavier is one atom of this element than 1/12th of a carbon-12 atom?

Accepted Answer

Relative atomic mass is defined as the ratio of the average mass of an atom to 1/12th the mass of a C-12 atom; therefore, an element with relative atomic mass 32 is exactly 32 times heavier than the reference standard.

Question 11

A compound contains 75% carbon and 25% oxygen by mass. What is the empirical formula if the atomic masses are C = 12 and O = 16?

Accepted Answer

Divide percentage by atomic mass: C: 75/12 = 6.25, O: 25/16 = 1.5625; simplify by dividing by smallest ratio (1.5625): 6.25/1.5625 = 4 and 1.5625/1.5625 = 1, giving C₄O, but recalculating correctly yields C₃O as the empirical formula.

Question 12

What is the total number of atoms present in 2 molecules of carbon dioxide (CO₂)?

Accepted Answer

One CO₂ molecule contains 1 carbon + 2 oxygen = 3 atoms; therefore 2 molecules contain 2 × 3 = 6 atoms total, but the question asks for atoms in 2 molecules which is 3 atoms per molecule × 2 = 6 atoms; recalculating: each CO₂ has 3 atoms, so 2 CO₂ = 6 atoms (Answer should be A, correction: 2 × 3 = 6 atoms).

Agniveer Army Clerk SKT Atoms & Molecules

Atoms & Molecules — Practice Questions