To know the molar mass of HCl we must add the atomic weight of each element, i.e. hydrogen and chlorine. The atomic weights can be found in the periodic table of the elements.
Hydrogen is found in group IA and chlorine is located in group VIIA.
The atomic weight of each element is:
Hydrogen: 1.008
Chlorine: 35.45
Molar mass HCl = 1.008 + 35.45 =36.458 g/mol