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Problem Set 2 Key CHEM103A Dr. Keller

# Problem Set 2 Key CHEM103A Dr. Keller - CHEMISTRY 103A...

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23 23 40.08 1 6.656 10 / 1 6.022 10 g mol g atom mol atom - = 23 20 6.656 10 741 4.932 10 1 g atoms g atom - - = CHEMISTRY 103A, Sections 7-10 Problem Set 2: 12 Points Answer Key Problem 1. What is the weight in grams of one mole of calcium (Ca)? Use the 4 sig fig value in your textbook for the atomic weight. 40.08 g Problem 2. What is the weight in grams of one atom of calcium ? The result from problem 1 is the mass of one mole of calcium. The units are formally g/mol. One mole of an element comprises Avogadros number of atoms. The units of the answer to problem 2 should be g/atom: Problem 3. What is the weight in grams of exactly 741 (counting number) calcium atoms ? This is the answer to problem 2 multiplied by 741. The units should be grams : Notice that since 741 is a counting number , i.e. exact, the answer should have four, not three, significant figures. Problem 4. How many moles of calcium is represented by the 741 (counting number) calcium atoms? One mole of calcium consists of 6.022 x 10 23 calcium atoms, so

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21 23 1 741 1.230 10 6.022 10 mol atoms mol atoms - = 4 40.08 741.0 2.970 10 1 g mol g mol l =
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