R. Kevin B. answered 10/30/19
Chemistry Teacher with years of experience in Microsoft.
Li2SO4 has 2 Li, 1 S, and 4 O's. To find the gram formula mass, you have to find the g/mol.
Basic process is # of each atom x its molar mass off of the periodic table. Then add up the masses. Li has 6.94 g/mol, S has 32.06 g/mol, and O has 16.0 g/mol.
So,
Li 2 x 6.94 = 13.88 g/mol
S 1 x 32.06 = 32.06 g/mol
O. 4 x 16.0 = 64.0 g/mol
Get the total: 13.88 + 32.06 + 64.0 = 109.94 g/mol. If you were going to 3 sig figs, this would give 110 g/mol which means 1 mol of Li2SO4 will have 110 g if you mass it on a triple beam balance (or other scale).
The gram formula mass is often called the molar mass, molecular mass, g/mol. They all mean essentially the same thing. A formula is an ionic compound. A molecule is a covalent compound. The molar mass of a ionic compound is therefore called the gram formula mass. The molar mass of a covalent compound is therefore called a molecular mass. The units for molar mass are g/mol, so you should be able to see how the words are interchangeable, and all describe the same process, just they can give you a little more specific information by using molecular or gram formula mass instead of molar mass.
A molar mass has the same numeric value as an atomic mass, but an atomic mass uses the units AMU or u. Amu are so small, they can't be measured reliably in a lab, so we use 12 g of C-12 as our basis for a bigger unit, the mole (abbreviated mol), so that we can then convert amu into grams (which CAN be measured in a lab). So, C-12 has 12 amu as its atomic mass (6 protons & 6 neutrons--each with 1 amu mass & 6 electrons with almost no mass), and C-12 has 12 g/mol as its molar mass (6.02 x 1023 x 6 protons, and 6.02 x 1023 neutrons to give 12 grams in 1 mole). NOTE: the masses listed on the periodic table are average masses for each element based on an element's isotopes, which is why they have decimal numbers and go to different levels of precision (decimal places). Avg. mass = % isotope A x mass isotope A + % isotope B x mass isotope B + % isotope C x mass isotope C + .....