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Willchemistry
Molar mass Li = 6.94 g/mol

1 mol --------- 6.94 g
2 mol --------- ?

Mass Li = 2 * 6.94 / 1

Mass Li = 13.88 g

hope this helps!
c5h11no2
You can look on the periodic table for the number of automic mass units in each element. 

1 mole Li = 6.94 amu
6.94 • 2 = 13.9 amu

There are 13.9 amu in 2 moles of Li