Water consists of hydrogen and oxygen…
We are given that the ratio of hydrogen to oxygen in water, by mass, is 2 : 16. Using our ratio multiplier we can re-write this as 2x : 16x. We can now use these expressions to determine how much oxygen is in 144 grams of water.
2x + 16x = 144
18x = 144
x = 8
Since x is 8, we know that there are 16 x 8 = 128 grams of oxygen in 144 grams of water.