Briefly explain the physical and chemical methods of determining the molecular weight/molar mass of high molecular compounds, with examples.

First off, molecular mass is the mass of a molecule. This is usually (pretty much always) taken relative to carbon-12 (1/12th of the mass of carbon-12). Molar mass is the mass (for practical purposes, we often use weight in place of mass, though it is most definitely not the same!) of one mole of substance.

Molecular mass is given in units (where 1 unit is 1/12 the weight of one atom of carbon-12). Molar mass is given in grams per mole (the SI unit is kg/mol, but that’s hardly ever used). The number for each is the same, but the unit is not. So carbon-12 has a molecular mass of 12 units, and a molar mass of 12 g/mol.

We have molecular masses listed for all the elements. And we know the relative abundance of the different isotopes on earth, so we have an average molecular mass. For all atoms, and most common compounds, you can just google the molar mass directly. For any other, you can calculate it by googling the molar mass of each atom, and adding the, all together.

So for HCl, you can either google HCl, or you can google H (1.008 g/mol) and Cl (35.453 g/mol), and add them. 1.008 + 35.453 = 36.461. So HCl has a molecular weight of 36.461 u, and a molar mass of 36.461 g/mol.