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He did use 8.00g at first and got the mass % composition with that, but he's also showing a different way to calculate the problem just in case we were only given a percentage. If it's a percentage, the easiest way to approach it is by imagining the sample mass as 100. g since your percentage and grams would then be the same.
It's easiest to visualize mass percentage by using 100g (because percents are obviously out of 100). I don't think it's necessary every time to convert to mass percentage if you already have the total grams (when I did the problem, I just used the 8 grams), but some people have been saying that it's nice to equalize the masses and Lavelle might award points/credit for doing it, even though it isn't really necessary.
Since the mass percentage compositions of the elements where calculated, you know how much of each element is in the compound as percentages. Using 100g instead of the actual 8.00g value makes it easier so you don't have to figure out the actual amounts of each element. The answer is not affected by this because all values are already calculated into percentages.
It isn't necessary to use 100g for that specific problem since the masses of each element were given, but with other questions where only the percent mass of each element is given, you can assume a 100g sample in order to convert percentages to grams. Hopefully this helps!
It is easier to use 100 g, so that the mass of each element equals the percent composition. If you're finding the empirical formula, assuming the percent composition is the amount of grams allows you to convert that to moles. It just makes the math easier. You still use the 8.00 g when you are finding the molecular formula, but using 100 g makes finding the empirical formula easier.
The 8.00g is used to calculate the percent mass of each element in Vitamin C with their given masses. Once those are found, it is easier to imagine the sample mass is 100g so that the percent mass composition of each element translates directly to masses (ex. 40.9% C = 40.9g C in a 100g sample). Then those masses can be used to find the moles of each element. While it is possible to calculate the moles using the original 8.00g, some find it conceptually easier to use a 100g sample.
The sample is still 8.00g but it is easier to imagine the sample as 100g because then you can use the percentages of C,H,O as the "mass" of each element. (percent of 100 so we imagine the sample is 100g) This way, you can immediately determine the moles of C,H,O. I hope this helps.
We assume that any given sample would be 100g because when finding the empirical or molecular formula, you must first begin with finding the percent mass composition. When you find the percent mass composition (say the value is 40.9% carbon) then you can convert that to grams much more simply by assuming there is 100 g of the whole substance (it would be 40.9 g). This method just makes it easier to figure out moles of any given element in the substance and makes the actual math part of the question easier. :)
You can use an arbitrary amount of grams to find mass percent. Using 100 grams just makes it easier since each 1% will be equal to 1 gram. However, you can't completely disregard the 8 grams because it will probably be needed to find the molecular formula, amount of grams of a specific element in a compound, etc.
Last edited by Joey_Okumura_1E on Mon Oct 05, 2020 5:24 pm, edited 1 time in total.
It doesn't matter what mass you use as long as the ratios of atoms match the mass percentage composition. You could use 8g or 100g and the answer you would get would be the same as long as the ratios are correct.
Dr. Lavelle first uses the 8g to find the mass percentage compositions of C, H, and O. He uses 100g for any instances where you are only given the mass percentages and not a sample mass. With only just the mass percentages, it's easier to imagine 100g as the sample mass so that the given mass percentages will be the same as they are in grams.
We use 100g because it makes it easier to imagine the sample mass. Since we already found the mass compositions of each element, we can just use those percentages and treat them as if they were grams. By converting them to grams we can then convert to moles to find the empirical formula.
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