Name: Valencies of Some Common Ions
Uploaded: 2019-07-19
Duration: 4 min 56 s
Description: Ionic compounds are composed of positively charged and negatively charged ions. Do you know what the charges on these ions are called? Let’s find out in this...

Question

Define molecular mass and formula unit mass.

Accepted Answer

Molecular mass of a substance (element or compound) is the average relative mass of its molecules as compared to that of an atom of $C - 12$ isotope taken as $12$ units. In other words, molecular mass of a substance (element or compound) represents the number of times the molecule of that substance is heavier than $\frac{1}{12} th$ of the mass of an atom of $C - 12$ isotope.

Hence, molecular masses are also expressed in atomic mass units (written as amu) or unified mass (written as u) on the atomic scale.

The formula unit mass of a substance is the sum of the atomic masses of all the atoms present in one formula unit of a compound.

For example, Formula unit mass of $NaCl$ = Atomic mass of $Na$ + Atomic mass of $Cl$

Formula unit mass of $NaCl = 23 + 35.5 = 58.5 u$

Since, molecular mass is calculated from the formula of the compound, thus, as a matter of fact, molecular mass of a substance is equal to its formula unit mass.

Define molecular mass and formula unit mass.

Important Questions on Atoms and Molecules

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