Please, What is the mass of oxygen in 72.0g of water? [ H= 1.0; O = 16.0]

Convert 72g of water into moles of water using molecular weights.

So water is H2O so add up those molecular weights (H=1 and O=16)

2(1)+(16) = 18 g/mol

Then convert so 72g / (18 g/mol) = 4 mol

Now you can convert mol of water to mol of oxygen. So 4 mol of water is 4 mol of oxygen. Then use oxygen molecular weight to find grams again.

4 mol oxygen * 16 g/mol = 64g of oxygen

If we were doing hydrogen instead of oxygen there would be 8 mol hydrogen in 4 mol of water (2 H’s in every H2O molecule) and since we have 74 grams and oxygen is 64 grams, Hydrogen should be 8 grams. Math to check below

8 mol hydrogen * 1 g/mol = 8g of hydrogen

It all adds to 72 so we are correct.