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:
- Determine the molecular formula of the compound.
- 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.
- Determine the atomic mass of each element in the molecule by referring to periodic table
- For each element in the compound, multiply its atomic mass by the number of atoms of that element.
- Molecular Mass =∑(Atomic Mass x No. Of Atoms)
Tips and Tricks:
- For elements where the subscript is missing, take the number of atoms as one.
- 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.
- 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