The molecular weight is 79.

The relative mass of n atom is 14.

The relative mass of H atom is 1.

The relative mass of O atom is 16.

The relative mass of C atom is 12.

There are 1 n atoms, 5 h atoms, 3 o atoms and 1 c atoms in ammonium bicarbonate (NH4HCO3).

Relative mass =14 *1+1* 5+3 *16+1*12 = 79.