In order to find the mass of a compound, we will usually use number of moles and molar mass, and in this question, we have the number of moles, which is 6.50E-1 (this is equal to 0.650 moles), and the molar mass is 7 g/mol, that means that we have 7 grams of Lithium for every mol of Lithium
7 g = 1 mol
x grams = 0.650 moles
x = 4.55 grams of Lithium
