Hello!
To start off, we must look at atomic masses. Atoms all have different weights, so we must first find hydrogen and oxygen's atomic masses.
Oxygen: 16.00 amu
Hydrogen: 1.01 amu
Now, moving on to the weight of water itself. Water has the formula of H20, with two hydrogen atoms and one oxygen. Therefore, add up the amus to get the weight of one molecule of water.
1.01 + 1.01 + 16.00 = 18.02 amu
Now, to see the ratio of each component. Since hydrogen weighs a total of 2.02 amu (1.01 + 1.01) in the entire atom, we can state that hydrogen makes up about 0.112 of the weight of water. Now apply that ratio to 16 g, and solve.
0.112x = 16
142.857143 = x
So therefore, about 143 grams of water are made when 16g of hydrogen reacts with excess oxygen.
Hope this helps!