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You have to find the mass of the elements that are given and convert the masses to percentages, assuming that it is out of 100%. It should always add up to 100 if you carry out the problem in this fashion.
Hope this helps!
Hope this helps!
Mass percentages should always add up to 100%. If yours don't, maybe you calculated something wrong, or maybe the question did not explicitly give you all the information. For example, if it is a combustion problem, the question might not have given you information about the O2 that is in the reactants, and you might have to calculate this mass percentage by yourself.
It should always add to 100% but I remember Dr. Lavelle stating that it is a combustion reaction and the mass percentages given do not add up to 100 then it is likely the oxygen that is being left out and will make up the difference.
There should be no loss of mass, as stated in the law of conservation of mass. If there were a case in which it does not add up to 100%, for instance, combustion, then it's safe to assume the missing portion is oxygen.
100 grams is merely a value that facilitates the conversion from mass percentage composition to actual mass composition. Rather than multiplying the percentage by 100 (and essentially only moving decimal points), you multiply by the specific given grams of the sample.
The mass percentages have to equal 100% because all the elements together must add up to the total molecule. However, you may be confused for the next step in solving an empirical formula problem, when we are asked to assume that the elements are part of a 100 gram sample. This simply just makes the math easier and quicker.
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