WHAT IS MOLECULAR MASS?
The atomic mass (ma or m) is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), the atomic mass is often expressed in the non-SI unit dalton (symbol: Da, or u) where 1 dalton is defined as 1/12 of the mass of a single carbon-12 atom, at rest.
This concept can be extended to calculate molecular masses. The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).
Illustration:
(a) Calculate the relative molecular mass of water (H2O).
(b) Calculate the molecular mass of HNO3.
Sol:
(a) Atomic mass of hydrogen = 1u,
oxygen = 16 u
So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is = 2 \(\times\) 1+ 1\(\times\)16 = 18 u
(b) The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N+ 3 \(\times\) the atomic mass of O = 1 + 14 + 48 = 63 u
Questions
1. Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Source: This topic is taken from NCERT TEXTBOOK
WHAT IS FORMULA UNIT MASS?
The Formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions.
For example, sodium chloride as discussed above has a formula unit NaCl.
Its formula unit mass can be calculated as - (1 \(\times\) 23) + (1 \(\times\) 35.5) = 58.5 u
Illustration:
Calculate the formula unit mass of CaCl2.
Sol:
Atomic mass of Ca + (2 \(\times\) atomic mass of Cl) = 40 + (2 \(\times\) 35.5) = 40 + 71 = 111 u
Questions
1. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
Source: This topic is taken from NCERT TEXTBOOK