Chem 1211 1st Edition Lecture 6 Outline of Last Lecture I Atomic Numbers II Isotopes III Law of Definite Proportions IV Ions and Ionic Compounds V Complex Ions VI Moles Outline of Current Lecture I Percent Composition II Formulas from Elemental Composition Current Lecture I Percent Composition elemental analysis a Comes from experiments b Gives all the elements and what percentage they occur in i Example In CH4 what is the percent Hydrogen by mass and the percent Carbon by mass 1 There is 1 Carbon at 12 01g 2 There are 4 hydrogens at 1 008 g a 12 01 4 032 gives a total mass of 16 042 b 12 01 16 042 percent carbon 75 c 4 032 16 042 percent hydrogen 25 ii OR we have an unknown compound and the only information we have is that it is 25 H mass and 75 C mass 1 We say that we have 100 g of the sample easier math a 0 25 g H g substance x 100 25 g H x 1mol 1 008 g 25 mol H These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute II b 75 g C g substance x 100 75 g C x 1mol 12 01 6 2 mol C c 25 mol H 6 2 mol C 4 H per each C d CH4 2 Take find grams of each element moles of each element c Concept Test Which molecules have the lowest and highest percent composition of oxygen by mass respectively i Acetic acid CH3CO2H ii Carbon monoxide CO iii Ethylene glycol HOCH2CH2OH 1 ANSWER ethylene glycol has the lowest percent composition of oxygen by mass and carbon dioxide has the highest percent composition of oxygen by mass a Reasoning the numerator is the amount of oxygen in the molecule in grams b The denominator is the total amount of the molecule in grams c Acetic 32 60 53 d CO 16 28 57 e Ethylene 32 62 52 d Ratios i If you find that the ratio of A to B is 1 5 1 there must be 3 moles of A to every 2 moles of B 1 You need an integer Formulas from elemental composition a Empirical formula the smallest whole number ratio i Ex CH2 b Molecular formula i Ex C2H4 c To go from empirical molecular formula you need a molecular weight i C2H4 CH2 1 Molecular weight C2H4 28 ii C5H10 CH2 1 Molecular weight C2H4 70 iii The molecular weight tells you how many times the empirical formula appears in the molecular formula d Law of Multiple Proportions one of two things must be true different molecules have different ratios of elements even if they have the same elements aka water and hydrogen peroxide or if they have the same elemental formulas those formulas represent different structures i Restated when two elements can combine to form more than one compound the amounts of one of them that combines with a fixed amount of the other will exhibit a simple multiple relation e Concept Test Two organic molecules are dimethyl ether CH3OCH3 and ethanol CH3CH2OH Which is the most accurate statement about the two molecules i Obey Law of Multiple Proportions because they have different structural formulas ii Violate Law of Multiple Proportions because they have identical elemental formulas C2H6O iii Are not subject to the Law of Multiple Proportions because they are organic iv This is a trick question so the formulas are not drawn accurately 1 ANSWER i they have different structural formulas f Concept Test A Rolaids antacid has 550 mg of calcium carbonate and 110 mg magnesium hydroxide per tablet What is the percentage of calcium by mass per tablet Mg 24 3 Ca 40 8 HO 17 01 CO3 60 01 i ANSWER 220 mg 660 mg 33 3 1 Reasoning denominator is the total mass of the tablet 550 110 2 The numerator is the mass of Calcium in the tablet Since Calcium makes up 40 of the 550 mg of calcium carbonate CaCO3 0 40 x 550 220 g Percent Purity i Percent purity mass of pure substance mass of sample x 100
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