Mass Composition from Molar Composition (Example)
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Mass Composition from Molar Composition (Example)

September 18, 2019


This is a problem where we are given the molecular formula for a compound, in this case DDT And the question is, what is the percentage composition by mass? So essentially, we want to covert moles of each species to mass and get a mass composition Since I have not looked at this problem before The way I would solve a problem like this is to write down information that I know. And I want the mass composition, which means I’m gonna covert the moles And I’m gonna take a basis that if I had 14 moles of carbon, 9 moles of hydrogen, and 5 moles of chlorine and what I’m going to need is the atomic masses I’m gonna look them up, so 14 moles of carbon there are 12.01 g/mol, so this gives me the grams of carbon 9 moles of hydrogen, and there’s 1.008 g/mol So this willl give me the grams of hydrogen And then chlorine, 35.45 g/mol if I do this multiplication grams of chlorin. So I’ve calculated the grams of each of the components. I can add this up to get the total mass This is 354.46 g total, and since I want the percentage composition by mass, for carbon, then I would take 168.14 g of carbon over the total, and then to get a percentage I multiply by 100, and 47.45%. I can do the same thing for hydrogen. I have 9.072 g of hydrogen over the total 354.46 g, multiplied by 100 and likewise for chlorine, 177.25 g of chlorine over the total of 354.46, again time 100 to make a percentage And so since the chemical formula is exact how many sigificant figure are just a function of how many significant figures I here, I’ll leave everything at two decimal places, this then is the mass percent composition for DDT

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