Calculate the molecular mass of the following : (i) H2O (ii) CO2 (iii) CH4
Method -1
(i)Molecular mass of H2O=2 × Atomic Mass of H+Atomic Mass of O=2+16=18 a.m.u
(i)Molecular mass of CO2 =Atomic Mass of C+2 × Atomic Mass of O=12+32=44 a.m.u
(i)Molecular mass of CH4 =Atomic Mass of C+4 × Atomic Mass of H=12+4=16 a.m.u
(i)Molecular mass of CO2 =Atomic Mass of C+2 × Atomic Mass of O=12+32=44 a.m.u
(i)Molecular mass of CH4 =Atomic Mass of C+4 × Atomic Mass of H=12+4=16 a.m.u
Method -2
(i) H2O:
The molecular mass of water, H2O
= (2 × Atomic mass of hydrogen) + (1 × Atomic mass of oxygen)
= [2(1.0084) + 1(16.00 u)]
= 2.016 u + 16.00 u
= 18.016
= 18.02 u
(ii) CO2:
The molecular mass of carbon dioxide, CO2
= (1 × Atomic mass of carbon) + (2 × Atomic mass of oxygen)
= [1(12.011 u) + 2 (16.00 u)]
= 12.011 u + 32.00 u
= 44.01 u
(iii) CH4:
The molecular mass of methane, CH4
= (1 × Atomic mass of carbon) + (4 × Atomic mass of hydrogen)
= [1(12.011 u) + 4 (1.008 u)]
= 12.011 u + 4.032 u
= 16.043 u
The molecular mass of water, H2O
= (2 × Atomic mass of hydrogen) + (1 × Atomic mass of oxygen)
= [2(1.0084) + 1(16.00 u)]
= 2.016 u + 16.00 u
= 18.016
= 18.02 u
(ii) CO2:
The molecular mass of carbon dioxide, CO2
= (1 × Atomic mass of carbon) + (2 × Atomic mass of oxygen)
= [1(12.011 u) + 2 (16.00 u)]
= 12.011 u + 32.00 u
= 44.01 u
(iii) CH4:
The molecular mass of methane, CH4
= (1 × Atomic mass of carbon) + (4 × Atomic mass of hydrogen)
= [1(12.011 u) + 4 (1.008 u)]
= 12.011 u + 4.032 u
= 16.043 u
