Find the percent by mass of lithium and oxygen in lithium oxide, Li2O.  

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t-nez eNotes educator| Certified Educator

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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