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Mass Questions

The quantity of matter in a sample is represented by its mass. Typically, mass is expressed in grams (g) and kilograms (kg).

Mass can also be thought of as a property of matter that causes it to resist acceleration. The greater an object’s mass, the more difficult it is to accelerate it.

Definition: A physical body’s mass corresponds to the amount of matter it contains. It’s also a measure of the body’s inertia, or the resistance to acceleration (velocity change) when a net force is applied. The gravitational attraction between two bodies is also determined by the mass of an object.

Mass Chemistry Questions with Solutions

Q1: What is the mass of hydrogen in terms of amu?

a) 1.0020 amu

b) 1.0180 amu

c) 1.0070 amu

d) 1.0080 amu

Answer: d) 1.0080 amu

Explanation: The mass of a hydrogen atom is 1.6736×10-24 g. When converted in terms of amu, 1.6736×10-24g should be divided by 1.66056×10-24 g. 1.6736×10-24 g/1.66056×10-24 g = 1.0078 amu = 1.008 amu. This is the process to measure any atomic mass in amu.

Q2: Nowadays, “amu” is replaced by ____

a) u

b) g

c) kg

d) am

Answer: a) u

Explanation: Nowadays, “amu” has been replaced by “u”. The Atomic Mass Unit used to be “amu,” however it was changed to “u” and is now known as Unified Mass. The mass of one nucleon is one unified atomic mass unit, which is also equivalent to 1 g/mol.

Q3: ______ is the sum of atomic masses of the elements present in a molecule.

a) Average atomic mass

b) Atomic mass

c) Gram formula mass

d) Molecular mass

Answer: d) Molecular mass

Explanation: The total of the atomic masses of the atoms in a molecule is called molecular mass. The amount of a substance with the same exact mass in grammes as the formula mass in amu is called gram formula mass. The “amu” is the standard unit of mass for measuring mass on a molecular or atomic scale.

Q4: What’s the molecular mass of carbon dioxide?

a) 43

b) 28

c) 40

d) 44

Answer: d) 44

Explanation: Carbon has an individual mass of 12 amu, while oxygen has a mass of 16 amu. CO2 is the chemical formula for carbon dioxide. As a result, there is one carbon and two oxygens. 12 (M.wt. of carbon) + 2 x 16 (M.wts. of oxygen) = 12 + 32 = 44. As a result, carbon dioxide has a molecular mass of 44.

Q5: Calculate the molecular mass of the sucrose (C12H22O11) molecule?

a) 343 amu

b) 342 amu

c) 341 amu

d) 340 amu

Answer: b) 342 amu

Explanation: Carbon has an individual mass of 12 amu, hydrogen has an individual mass of 1 amu, and oxygen has an individual mass of 16 amu. However, sucrose has 12 carbons, 22 hydrogens and 11 oxygens, that is 12 x 12 + 22 x 1 + 11 x 16 = 342 amu.

Q6: What’s the formula mass of NaCl?

a) 23 u

b) 35.5 u

c) 58 u

d) 58.5 u

Answer: d) 58.5 u

Explanation: We can only calculate the formula mass of Cl because it can’t exist in the solid state in NaCl. Sodium has a mass of 23 units, while chlorine has a mass of 35.5 units. Individual masses, 23 + 35.5 = 58.5 u, are added together in the formula mass of Sodium chloride.

Q7: What is wrong in saying ‘one mole of nitrogen’?


The sentence is unclear as to whether we’re talking about nitrogen atoms or molecules. ‘One mole of nitrogen atoms’ or ‘one mole of nitrogen molecule’ should be used instead.

Q8: How many times heavier is one atom of carbon than one atom of oxygen?


Atomic mass of carbon = 12 u

Atomic mass of oxygen = 16 u

Thus, one atom of carbon is 12u/16u = 3/4 times heavier than one atom of oxygen.

Q9: What is meant by the term chemical formula? Give examples.


A compound’s chemical formula is a symbolic representation of its composition and the number of atoms in a single molecule of a pure substance, which can be an atom or a compound.

For example, the Chemical formula of ammonia is NH3, water is H2O, and carbon dioxide is CO2.

Q10: Define one mole, and illustrate its relationship with the Avogadro constant.


A mole is a quantity in number equal to the atomic or molecular mass in grammes of any species (atoms, molecules, ions, or particles).

With a value of 6.022 x 1023, the number of particles (atoms, molecules, or ions) present in 1 mole of any substance is constant. This number is termed the Avogadro constant or Avogadro number.

Q11: What is the mass of the following:

(a) 1 mole of nitrogen atoms?

(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?

(c) 10 moles of sodium sulphite (Na2SO3)?


(a) 1 mole of nitrogen atoms

= 1 x gram atomic mass of nitrogen atom

= 1 x 14 g = 14 g

(b) 4 moles of aluminium atoms

= 4 x gram atomic mass of aluminium atoms

= 4 x 27 g = 108 g

(c) 10 moles of sodium sulphite (Na2SO3)

= 10 (2 x gram atomic mass of Na + 1 x gram atomic mass of sulphur + 3 x- gram atomic mass of oxygen)

= 10(2x23g + 1 x32g + 3x16g)

= 10 (46 g + 32 g + 48 g)

= 10 x 126 g = 1260 g

Q12: Calculate the molecular masses of H2, O2, CO2, CH4, NH3, CH3OH.


The molecular mass is the total sum of the masses of the atoms or components in the molecule.

(i) Molecular mass of H2 = 2 x Atomic mass of H

= 2 x 1 = 2 u

(ii) Molecular mass of O2 = 2 x Atomic mass of O

= 2 x 16 = 32 u

(iii) Molecular mass of CO2 = Atomic mass of C + 2 x Atomic mass of O

= 12 + 2 x 16 = 44 u

(iv) Molecular mass of CH4 = Atomic mass of C + 4 x Atomic mass of H

= 12 + 4 x 1 = 16 u

(v) Molecular mass of NH3 = Atomic mass of N + 3 x Atomic mass of H

= 14 + 3 x 1 = 17 u

(vi) Molecular mass of CH3OH = Atomic mass of C + 4 x Atomic mass of H + 1 x Atomic mass of O = 12 + 4 x 1 + 16 = 32 u

Q13: Give the postulates of Dalton’s atomic theory.


  • Every element is made of atoms, which are extremely tiny particles.
  • The mass and characteristics of all atoms of a specific element will be identical. Distinct chemical elements have varied types of atoms, as well as different masses for those atoms.
  • Atoms can’t be created, destroyed, or converted into other elements’ atoms.
  • When atoms of several elements combine in small whole number ratios, compounds are created.
  • The relative number and types of atoms in a compound are constant.

Q14: When 3 g of magnesium is burned with 2 g of oxygen, the result is 5 g of magnesium oxide. When 3 g magnesium is burned in 5 g oxygen, how much magnesium oxide is produced? Which chemical combination law will determine your reaction? State the law.


When 3 g of magnesium is burned with 2 g of oxygen, the result is 5 g of magnesium oxide. Magnesium and oxygen are mixed in a 3:2 ratio to produce magnesium oxide.

When 3 g of magnesium is burned in 5 g of oxygen, 5 g of magnesium oxide is produced, while the remaining oxygen is wasted.

It is controlled by the law of definite proportion.

It states that the elements of a chemical substance are always present in specific mass proportions.

Q15: (a) Why does not atomic mass of an element demonstrate the actual mass of its atom?

(b) “An element’s atomic mass is a fraction.” What does it indicate?


(a) Atoms of distinct elements are quite small and their actual masses are incredibly tiny. The relative atomic mass of the element is used to solve this problem. Hydrogen has a relative atomic mass of 1 u, which corresponds to a gram atomic mass of 1 g.

(b) If the atomic mass of an element is in fraction it implies that it exists in the form of isotopes. The atomic mass of a given element is the sum of the atomic masses of its isotopes, and it is usually expressed as a fraction.

Practise Questions on Mass

Q1: What is the abbreviation of amu?

a) Atomic matter unit

b) Atomic mass unified

c) Atomic mass unit

d) At mass unity

Q2: A sample of carbon that contains 70% carbon-12 and 30% carbon-14. What do you think is the average atomic mass of this sample?

a) 14.5

b) 14.14

c) 14

d) 12

Q3: Give an example to show the law of conservation of mass applies to physical changes also.

Q4: Differentiate between the actual mass of a molecule and gram molecular mass.

Q5: Which of the following are tri-atomic and tetra-atomic molecules?

CH3Cl, CaCl2, NH3, PCl3, P2O5, H2O, C2H5OH

Click the PDF to check the answers for Practice Questions.
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