Hello, John,
Cheer up! Chem can be a pain, but we'll take this step by step to demonstrate the main ideas behind balanced equations and these types of questions. I'm not sure which aspect is the most confusing, so I'll move through the problem fairly quickly, but please let us know which step is causing more confusion.
Making water from it's elements can only mean H2 and O2, so let's make a balanced equation for
H2 + O2 = H2O
We need to balance this equation: we have 2 oxygen atoms coming in, but only 1 going out as H2O. Use pencil when balancing equations - we may need to make several adjustments in the process. we need to add a coefficient of 2 to the H2O to use up the second O atom coming in.
H2 + O2 = 2H2O
That works of oxygen, but now we're short 2 H atoms. So add a 2 in front of hydrogen:
2H2 + O2 = 2H2O
Now the equation is balanced. The same atoms coming in with the reactants are all accounted for with the 2 H2O molecules.
Now we can determine the mass of O2 needed to make 54 grams of water.
Determine the moles of water in 54 grams. The molar mass of H2O is 18 grams/mole (just add the atomic weights of each atom in a molecule to find the molar mass, grams/mole).
Moles water = 54 grams/18 grams/mole or 3.0 moles of water.
The balamnced equation says we need 2 moles of H2 for every 2 moles of H2O produced. That's a mole ratio of 1:1. We'll need 3 moles of hydrogen, H2, to make 3 moles of H2O.
The questions wants the mass of hydrogen, in grams. Hydrogen's molar mass is 2 grams/mole. So we'll need
(3 moles H2)*(2 grams H2/moles H2) = 6.0 grams H2
Please ask if something doesn't make sense.
Bob
