In order to find the amount of zinc obtained, first we need to set up the equation properly balanced:
2 ZnO -> 2 Zn + O2
Now we have 25 grams of ZnO, we need to find out how many moles does it represent, we will do that by using the molar mass of ZnO, which is 81.38 g/mol
81.38 g = 1 mol
25 g = x moles
x = 0.307 moles of ZnO
By looking at the reaction we see that ZnO and Zn have the same molar ratio, which means that if we use 2 moles of ZnO in the reaction, we will end up with 2 moles of Zn as product, making it a molar ratio of 2:2, therefore:
2 ZnO = 2 Zn
0.307 ZnO = x Zn
x = 0.307 moles of Zn in the reaction as well
Now to find the mass of Zn, we will also use its molar mass, which is 65.38 g/mol
65.38 g = 1 mol
x grams = 0.307 moles
x = 20 grams of Zn will be obtained from 25 grams of ZnO