Use your knowledge of the law of conservation of mass to answer the following question: In the lab, a student reacts lithium with oxygen to produce lithium oxide. (Lithium + Oxygen -----> Lithium Oxide) How much lithium will be needed to react with 5.4 grams of oxygen to produce 24.6 grams of lithium oxide?

Answer 1

write the balanced equation
see how many moles of Li₂O per mole of Li
convert grams to moles.
do the math.

Answer 2

The numbers in the problem are not correct; therefore, using the law of conservation of mass doesn't work. For example, 5.4 g O2 will produce 10.08 g Li2O. If the problem were quoted properly for conservation of mass, it would be using 5.4 g O2 how much Li would be required to produce 10.08 g Li2O. Then you would do this:
..................4Li + O2 ==> 2Li2O
.....................x.....5.4.........10.08
Then x + 5.4 = 10.8 and
x = 10.8 - 5.4 =4.68
Use the method shown by oobleck, although not using the law of conservation of mass, that 4.68 is correct; i.e.,
mols O2 = 5.4/32 = ?
mols Li = 4*mols O2
g Li = mols Li x atomic mass Li = 4.68