To find the percent by mass of lithium and oxygen in Li2O you need to know the molar mass of both elements:
Li = 6.94 g/mole
O = 16.00 g/mole
The mass of one mole of Li2O is the sum of the mass of two moles of lithium atoms plus one mole of oxygen atoms:
2(6.94 g/mole + 16.00g/mole =
13.88 + 16.00 = 29.88 g/ mole
The percent of each element is its mass divided by the total:
O = 16.00/29.88 = 0.5355 = 53.55 %
Li = 13.88/29.88 = .04645 = 46.45%
Since Lithium appears twice in the formula it's mass is multiplied by 2 then divided by the mass of the formula.
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