Hello, Emily,
We need to start with a balanced equation. Leaving out the electrons, we get:
2H2O = 2H2 + O2
Both the oxygen and hydrogen are diatomic, so they have to be written as H2 and O2. We need two H2O to provide two oxygens, which forces the H2 to double since we now have 4 hydrogens on the reactant side..
The question is asking whether we will wind up with more or less than 10 grams of products. Now is a good opportunity to tell you something: DO NOT do what I just did. I assumed I knew how the answer the question before I really looked at what it was asking. So I wrote a balanced equation to start the answer, but I needn't have bothered.
The law of conservation of mass tells us the answer far more directly: The mass of products will be the same as the mass of reactants. There is no loss or gain in a chemical reaction. Read the question before starting the answer, Bob. [Please note I said "chemical" reaction. Nuclear reactions do involve some change in mass as it is converted into energy. a longer, but fascinating, explanation that hopefully will be coming in a later chapter]
I hope this helps,
Bob
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Just for fun, since I have the equation, let's try an example. Start with 36 grams of water. That is two moles of H2O. 36g/(18g/mole).
The equation tells us we will get 2 moles of H2 and 1 mole of O2 from our 2 moles of water. I can convert moles into grams by dividing each by their molar mass:
2 moles H2 = 2 moles/(2 grams/mole) = 4 grams H2
1 mole O2 = (32 grams/mole) = 32 grams O2
They add to 36 grams, the same mass as the starting mass of water.
But someone brighter than I took that realization and made it into the Law of Conservation of Mass. There are other conservation laws for energy and momentum. Handy to know for those with the patience to read the entire question before answering.
