To find the molecular mass of ammonia, we have to know its molecular formula.
The molecular formula of ammonia is NH₃, it means that it contains nitrogen and hydrogen.
To calculate the molecular mass of this compound we have to look for the atomic masses of N and H (you can do it using your periodic table or Internet), once we have them, multiply each atomic mass by the subscript of the element in the molecular formula of the compound. In this case, multiply the atomic mass of N by 1 and the atomic mass of H by 3. Finally, add these products.
The atomic mass of N is 14g/mol and the atomic mass of H is 1g/mol.
Find the molecular mass this way:
It means that the molecular mass of ammonia is 17g/mol.