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# c101_f10_1013 - Percent composition(mass percent percent...

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Percent composition (mass percent, percent composition by mass) Mass % = mass of component in substance (grams) x 100 mass of substance (grams) What is the mass % composition of 10 g water? MM (H 2 O) = 18.0152 g/mol MM (H) = 1.0079 g/mol MM (O) = 15.9994 g/mol %H in water = 11.21% H %O in water = 88.79% O If you have 18.015 g of water, what is the mass % composition of water? Does mass % composition of a substance depend on the amount of substance you have? Why?

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Chemistry 101 Fall 2010 2 We went from chemical formula (H 2 O) to percent composition. We can also go from percent composition to the chemical formula. You have 100 grams of a compound, and you find out it has a mass % composition of: 75.7% C 8.8% H 15.5% O. How many moles of C, H, and O do you have?
Chemistry 101 Fall 2010 3 Mass % composition of: 75.7% C, 8.8% H, and 15.5% O. yields these molar ratios … mol C : mol H : mol O, which is the same as saying … atoms C : atoms H : atoms O. Atoms do not combine as fractions or pieces (0.30, 0.7, 0.969), so we must scale up to reach whole atoms (whole numbers).

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