Molecular mass

Molecular mass

source : BOC Sciences

Molecular mass-

“molecular mass is a number which indicates how many times one molecule of a substance is heavier in comparison to 1/16^th of the mass of oxygen-16 or 1/12^th of the mass of one atom of carbon – 12″.

Molecular mass = mass  of one molecule of the substance / (1/12^th ) part of the mass of one atom of carbon-12

Mass of one molecule is equal to sum of the masses of atoms present in a molecule.

Ex- one molecule of HNO₃ consists of 1 atom of hydrogen , 1 atom of nitrogen and 3 atoms of oxygen.

Molecular mass of HNO₃ = 1 x 1.008 + 14.00 + 3 x 16.00= 63.008 amu

Gram molecular mass or Gram molecule-

“Gram molecular  mass(weight) of a substance  is the mass of Avogadro number  ( 6.023 x 10²³ ) of molecules  in grams.”

Ex –

mass  of 1  oxygen molecule (O₂) = 32 amu = 32 x 1.66 x 10^-24 gm

Mass  of 6.023 x 10²³ molecules  of oxygen(O₂) = 32 x 6.023 x 10²³ x 1.66 x 10^-24 = 32 gm.

Hence, Gram molecular mass of oxygen(O₂)  = 32 gm.

No. of gram molecules( moles) = mass of substance  in gm / molecular  mass of substance in gm.

Question 1) What is the molecular weight of a substance , each molecule of which contains 9 carbon atoms, 13 hydrogen atoms and 2.33 x 10²³ gm of other component ?

Solution )

The molecule has carbon , hydrogen and other component.

weight of 9 C = 12 x 9 = 108 amu

weight of 13 H = 13 x 1= 13 amu

weight of other component= 2.33 x 10²³/1.66 x 10^-24= 14.04 amu

Total weight of one molecule = molecular weight = 108 + 13 + 14.04 = 135.04 Ans.

Question 2) How many molecules are in 5.23 gm of glucose ?

Solution ) Formula of glucose is C₆H₁₂O₆.

molecular weight of glucose = 12 x 6 + 1 x 12 + 16 x 6 = 180

180 gm glucose has = 6.023 x 10²³ molecules

5.23 gm glucose has = 5.23 x  6.023 x 10²³ /180

no. of molecules = 1.75 x 10²²  Ans.

Question 3) How many molecules of water are present in one ml. of water ?

Solution )

mass of water = v x d

d(density) of water =1 gm/cc

v= 1 ml

mass of water = 1 x 1= 1 gm.

18 gm water has = 6.023 x 10 ²³ molecules

1 gm water has = 6.023 x 10 ²³ x 1 / 18

no. of molecules = 3.34 x 10²²

Question 4) How many molecules of water are present in one ml. of water vapours at NTP  ?

Solution )

v = 1 ml = 1 x 10^-3 litre

22.4 litre of water vapours at NTP has = 6.023 x 10 ²³ molecules

1 x 10^-3 litre of water vapours at NTP has = 6.023 x 10 ²³  x 1 x 10^-3 /22.4 molecules

no. of molecules = 2.69 x 10¹⁹ Ans.