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Chemical Quantities (the MOLE) Chapter 10

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Counting Units How many is a dozen? How many does the word “couple” stand for? How many sheets are in a ream of paper? Answers: 12, 2, 500

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Chemist’s “Dozen” Chemists also have a counting unit for a certain number of things A mole = Avogadro’s number of things 1 mole = 6.02 x 10 23 things –Could be atoms, ions, particles, molecules, etc. How big is that number? –See Sheet: A mole is a lot of things

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Sample Problem How many atoms are contained in 4.5 moles of copper? How many moles are in 8.3 x 10 24 molecules of water? Answers: 2.71 x 10 24 atoms, 13.8 moles

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Masses and Moles If you had a dozen bowling balls and a dozen ping pong balls, which would have more mass? Why? The bowling balls would have more mass because each individual bowling ball has more mass than a ping pong ball.

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Mass of Elements and Moles Which would be heavier: a mole of copper or a mole of lead? Why? A mole of lead would be heavier because an atom of lead is heavier than an atom of copper (64 amu vs. 207 amu)

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Carbon-12 and Avogadro’s Number Avogadro’s number is related to the mass of an element, specifically carbon (the isotope carbon-12) Used exactly 12 grams of carbon and determined the number of atoms in that sample There are 6.02 x 10 23 atoms in 12 grams of carbon (1 mole)

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Definition of Mole A mole is the amount of matter that contains as many objects (atoms, molecules, etc.) as the number of atoms in exactly 12 g of 12 C. –Mole connects amu & gram = “Chemists dozen”

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Molar Mass How do you determine moles if you have a compound? What is the mass of 1 mole of NaCl? Answer: 58 g/mol Molar Mass: the mass of one mole of a substance (element or compound)

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Sample Problem- molar mass What is the mass of one mole of –water? –Carbon dioxide? –C 6 H 12 O 6 (glucose) –Ca(NO 3 ) 2 –Answers: 18 g/ mol, 44, 180, 164

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Sample Problem- moles to mass How many grams of sodium chloride (NaCl) did you use if you put 0.250 moles of it on your French fries? Answer: 14.5 g

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Sample Problem – mass to moles How many moles of helium are in a balloon that contains 4.5 g of helium? Answer: 1.1 moles

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Molar Volume Can you determine moles from the volume of a substance? –For solids and liquids: must use density –For gases: related to the temperature and pressure of the gas.

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Avogadro’s Hypothesis Equal volumes of gases at the same temperature and pressure contain the same number of molecules All gases behave the same way, no matter the chemical identity

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Standard Temp and Pressure Because the volume of a gas depends on the temp and pressure of the gas, a standard reference is necessary STP: Standard Temperature and Pressure –Std Temp: 0 o C or 273 K –Std Press: 101.3 kPa or 1 atm 1 mole of a gas at STP = 22.4 L

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Sample Problem – Volume to moles How many moles of helium are in a balloon has a volume of 6.40 L? (at STP) Answer:.286 moles

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Study Buddy Review What is Avogadro’s Number? What is a mole? What is molar mass? How do you find the molar mass of a compound? What is STP?

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Percent Composition

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Penny Composition Remember: a penny’s composition has changed through the years –Most recent change 1982 If a post-1982 penny is 2.500 g and it has 0.0625 g copper, what percentage is copper? What percentage is zinc? Answers: 2.50 % Cu, 97.50 % Zn

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Percent Composition mass of each element in compound compared to the entire mass of the compound, multiplied by 100%

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Sample Problem What is the percentage composition of copper in chalcocite, Cu 2 S? Answer: 80 %

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Consider these C 6 H 12 O 6 glucose (blood), fructose (corn syrup) C 5 H 10 O 5 ribose (RNA), xylose What do the formulas have in common?

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Empirical formulas CH 2 Osmallest whole number ratio Carbohydrates: C n (H 2 O) n Empirical Formulas: smallest whole number ratio of the atoms in a substance

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Molecular Formulas Molecular Formula: whole number multiple of empirical – –tells the actual number of atoms in a substance as it exists in nature –Can be the same as the empirical formula

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