How many grams of oxygen are formed when 6.21 moles of koh are formed?
4 ko(s) + 2 h2o(l) → 4 koh(s) + o2(g)

From the given balanced chemical equation, it can be seen that for every 4 moles of KOH formed, only 1 mole of O2 is formed. From this ratio extracted from the equation, 
                   moles O2 = (6.21 moles) x (1 mole O2/4 moles KOH) 
                   moles O2 = 1.5525 moles O2
Then, we determine the mass in grams by multiplying the calculated value with the molar mass of oxygen, equal to 32. 
 The answer is therefore equal to 49.68 grams.