A basic Chemistry question, PLEASE HELP!

Question

I have the answer, but need to know HOW to get the answer.

Given the balanced equation, 2 Zn O2 -----> 2 ZnO How many grams of oxygen gas must react to give 1.28 g. of ZnO? The answer is supposed to be 0.252 g O2.

Answer 1

These stoichiometry problems are always done in four steps.

1. Balance the equation. (it is)

2. convert g of the given substance to moles.

....1.28 g ZnO x (1 mole ZnO / 81.4 g ZnO) = 0.0157 moles ZnO

3. Using the balanced equation, convert moles of the given substance (found in step 2) to moles of the desired substance.

0.0157 moles ZnO x (1 mole O2 / 2 moles ZnO) = 0.00786 moles O2

4) Convert moles of the desired substance (found in step 4) to grams.

0.00786 moles O2 x (32.0 g O2 / 1 mole O2) = 0.252 g O2

Answer 2

Moles ZnO = mass / molar mass = 1.28 g / 81.39 g/mol = 0.0157

The ratio between ZnO and O2 is 2 : 1

Moles O2 = 0.0157 / 2 =0.00785

Mass O2 = 0.00785 x 32 g/mol =  0.252 g