Equilibrium occurs when forward and reverse directions of a reversible reaction occur at the same rate so there is no overall change in the amounts of reactants and products.
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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
The answer is 0.0430 following the rules of dividing and multiplying significant figures.