34 g of O2 are reacted with excess Cs, causing a production of 199 g of Cs2O. What is the percent yield of this reaction?
2 Cs + O2 ---> Cs2O

33.23 %

Explanation:

First we convert 34 g of O₂ into moles, using its molar mass:

Then we convert O₂ moles into Cs₂O moles, using the stoichiometric coefficients of the balanced reaction:

Now we convert 2.125 moles of Cs₂O into grams, using its molar mass:

598.85 g is the theoretical yield. Finally we proceed to calculate the percent yield:

the answer is 75.0%.