What Will Be Given: Atomic Mass of Each Element in The Compound

What To Calculate: Molecular Mass / Molar Mass

SI Unit: kg/mol

Explanation :

The molecular mass(or weight) or molar mass is the total mass of the elements in the compound. It is the sum of atomic masses of each atom in the molecule.

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Steps to Calculate:

1. Determine the molecular formula of the compound.
2. Each element’s subscript in the molecular formula represents the number of atoms of that element. For example, H2O means two atoms of Hydrogen and one Atom of Oxygen.
3. Determine the atomic mass of each element in the molecule by referring to periodic table
4. For each element in the compound, multiply its atomic mass by the number of atoms of that element.
5. Molecular Mass =∑(Atomic Mass x No. Of Atoms)

Tips and Tricks:

1. For elements where the subscript is missing, take the number of atoms as one.
2. If the four choices given in the question are whole numbers, neglect the decimal digits of the atomic mass of the element in your calculation.
3. Use the given atomic mass in the problem and correct the significant digits based on the smallest number of significant figures in the atomic mass values of the elements.

Example -1

Find the molecular mass of Ammonia

Molecular Formula = NH₃

Atomic Mass of Hydrogen (H) = 1
Atomic masses of nitrogen (N) = 14

Molecular mass = (1 x 14) + (3 x 1)
= 14 + 3
= 17

Example -2

Find the molecular mass of Calcium Phosphate

Molecular Formula = Ca₃(PO₄)₂

Atomic Mass of Ca = 40
Atomic Mass of P = 31
Atomic Mass of O = 16

Molecular Mass of Ca₃(PO₄)₂ = (40 x 3) + (31 x 2) + (16 x 8)

=120 + 62 + 128

=310

Related Questions

1. The weight of one molecule of a compound C₆₀H₁₂₂ is
2. The number of oxygen atoms in 4.4 g of CO2
3. 0.45 g acid of molecular weight 90 was neutralise by 20 ml of 0.5 N KOH