Exercise 4 | The periodic table and Chemical Periodicity

1) Calculate the empirical formula of vitamin C where the compound is 40.9% C, 54.4% O and 4.58% H by mass.

2) Given that the molar mass of this vitamin is 176 g.mol-1, determine its molecular formula

1) The empirical formula of vitamin C is CXOYH⇒ MCxOyHz = X MC + Y MO + Z MH = 12.0 X + 16.0 Y + 1.01 Z

%C = 12.0 X / MCxOyHz = 0.409
%O = 16.0 Y / MCxOyHz = 0.544
%H = 1.01 Z / MCxOyHz = 0.0458

If  X = 1, MCxOyHz = 29.3 g.mol-1 and Y = 1 and Z = 1.33.

X, Y and Z must be the smallest whole number possible, so X=3, Y=3 and Z=4

The empirical formula of vitamin C is C3O3H4.
 

2) The molar mass of C3O3H4 is 88.0 g.mol-1

The molar mass of this vitamin is 176 g.mol-1

176/88 = 2, the molecular formula of vitamin C is C6O6H8