Define the terms theoretical yield, actual yield, and percent yield. Why is the actual yield in a reaction almost always less than the theoretical yield? Can a reaction ever have 110% actual yield?

Actual yield in a reaction is almost always less than the theoretical yield, primarily because losses of the substances involved may occur anywhere in an experiment. Otherwise, there can be so many possibilities that can be reasoned out depending on the reaction. For example, the possible decreasing exposure of the reactants with each other as the amount of products increase can be a reason, but this may not be applicable to all reactions. Temperature, water content, and the nature of the reactants may also play a significant factor to such occurrence.

Of course, a "true" actual yield can NEVER be greater than the "true" theoretical yield in a reaction ( this is the answer to the last question you posed ). What I mean by a "true" theoretical yield is the theoretical yield wherein you have accounted ALL of the possible occurrences in your reaction. For example, if the contaminants in your reaction container can react with your desired product to produce the same reactant again, and you didn't know that, then it's possible that the theoretical yield you calculated will not be the same as the "true" theoretical yield that should be considered.

If your calculations were correct, then there is no way you could have greater actual yield than the theoretical yield, because that scenario violates the Law of Conservation of Mass , which can be stated as:
"The total mass of the reactants must be equal to the total mass of the products."

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Also, experimental errors can occur, and if you successfully remove all of them, that's the time you would have your "true" actual yield. For example, you weighed what seemed to you a dry filter paper containing your precipitate, but it may actually have some more water in it that you still haven't removed. That can contribute to more weight of the sample. Errors from the apparatuses and instruments you have used but you didn't notice can also add up, making it possible to get 110% actual yield.