How many atoms are in 766.3 grams of Li?

1 mol of any substance is made of 6.022 x 10²³ units, these units could be atoms making up elements or molecules making up a compound.

In this case. 1 mol of Li is made of 6.022 x 10²³ atoms of Li

The molar mass of Li is 6.94 g/mol

Therefore mass of 1 mol of Li is 6.94 g

In 6.94 g of Li there are - 6.022 x 10²³ atoms

Then in 766.3 g of Li there are - 6.022 x 10²³ atoms / 6.94 g x 766.3 g = 665 x 10²³

There are 6.65 x 10^25 atoms of Li