What is the molar mass of lithium oxide, #Li_2O#?

#color(purple)"Let's use this equation to answer the question:"#

atomic weight of element #xx#number of atoms given by subscript #=# molar mass

First, you want to have your handy dandy periodic table available so you can determine the atomic weight of Li and O:

Li has an atomic weight of 6.94 g/mol
O has an atomic wight of 16.00 g/mol

Based on the chemical formula, we have 2 atoms of Li, so the atomic weight of Li has to be multiplied by 2 to obtain 13.88 g/mol.

You have 1 atom of O, so the atomic mass would just be 16.00 g/mol

Now you want to add the mass of each atom together to obtain the molar mass of the entire compound:

#color(red) ("13.88 g/mol + 16.00 g/mol = 29.88 g/mol")#

#Li_2O# is composed of Lithium and Oxygen atoms.

If you refer to your periodic table, the atomic weight is the molar mass of each element.

The molar mass of Lithium is 6.9 g/mol and Oxygen 16.0 g/mol.

Since there are 2 atoms of Lithium - if you look closely in #Li_2O#,
the molar mass of lithium #Li_2O# is 6.9 #xx# 2 = 13.8 g/mol.

Add the molar masses of Lithium and Oxygen together:
13.8 g/mol + 16.0 g/mol therefore the molar mass of Lithium oxide is 29.8 g/mol.