|MadSci Network: Chemistry|
%-Yield is a theoretical number that represents the result you should get when a chemical reaction is complete. While the math involved and the chemical equations are fairly stright forward when percent yields are calulated you are assumimg your measurements for the amounts of reactants and the amounts of products are accurate, also you are assuming the product you captured, filtered etc. is pure. All of these can lead to error. All measurements have error in them, and sometimes these errors can increase your yield number or decrease your yield number. In your case having water vapor in with the Hydrogen Gas is likely. re-examine your procedure and see if this is a possibility. Good Luck.
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