What is the gram formula mass of K2CO3 ?

The gram formula mass of a compound is nothing but the molar mass.

The molar mass of K2CO3 (Potassium Carbonate) is given by

Atomic weights of individual elements are

K (Potassium) = 39 g

C (Carbon) = 12 g

O (Oxygen) = 16 g

Hence, that of K2CO3 = 39(2) + 12 + 16(3)

= 78 + 12 + 48

= 138.