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...
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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