Chapter 6 Chemistry Notes

From the chemical formula c2h6o2 2 c atoms or 2 mol c

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Unformatted text preview: empirical formula gives the chemical formula with the smallest whole-number ratio of the elements of the compound. How many moles of each? The sample has 38.7 g C, 9.7 g H, and 51.6 g O. 1mol C 38.7 g C = 3.23 mol C 12.0 g C 1 mol H 9.7 g H = 9.6 mol H 1.01g H To start let’s assume a 100. g sample. (This will make the math in the 1st step easy) 2. Since the # of atoms are integers, divide by the smallest value and find the smallest integer values. 1O:1C:3H CH3O is called the empirical formula Notice how we used the molecular formula and the mole to get to this point. Empirical vs Molecular Formula A sample is composed of only 38.7% C, and 9.7% H, & ?% O, what is the chemical formula of the compound? 1. A mass percent is just the fraction mass of an element divided by the total mass of the molecule times 100. (Fraction*100=%) From previous result: O + C + H = 240. + 180. + 45. = 465 g EG 240. g O *100% = 51.6% O 465 g EG 180. g C *100% = 38.7% C 465 g EG 45.g H *100% = 9.7% H 465 g EG Conservation of mass check: O + C + H = 240. + 180. + 45. = 465 g √ 1 mol O 51.6 g O = 3.23 mol O 16.0 g O Often this information is express as a mass percent. Mass ? 1 mol EG 465 g EG 62.1g EG We have determined the elemental composition by mass of a substance using a macroscopic sample and a chemical formula. 3.23 mol O =1 O 3.23 mol 3.23 mol C ≈1 C 3.23 mol 9.6 mol H ≈3 H 3.23 mol (We encounter a similar idea when we used the smallest neutral fragment for the formula unit of an ionic substance). The empirical formula results from the use of macroscopic composition data working down toward molecular composition. The problem is that at the macroscopic level the sample is homogenous (a uniform mixture of elements). It is not until the molecular level that the individual constituents can be resolved. Some experiments are capable of finding the molecular mass, the mass of 1 molecule (or molar mass, g/mol, of molecules). Only with this addition information can the molecular formula be found. The molecular formula gives the elemental composition of the molecul...
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This note was uploaded on 02/04/2014 for the course CHEM 004:007 taught by Professor Russellarsen during the Fall '13 term at University of Iowa.

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