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General Chemistry Zumdahl Chapter 9 Notes Chemical Quatities Week of 2/9/15, expected test date 2/19/15 Standard: HS-PS1-7 Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

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What is the goal? Your test will essentially be this one question… How many molecules of water are formed if 453 grams of oxygen gas reacts with an excess of hydrogen gas?

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Step 1: Write out the equation Hydrogen (H 2 ) gas reacts with oxygen (O 2 ) gas to form water (H 2 O) liquid? H 2 (g) + O 2 (g) H 2 O(l)

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Step 2: Balance equation H 2 (g) + O 2 (g) H 2 O(1) 2H 2 (g) + O 2 (g) 2H 2 O(l)

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Step 3: Find moles of given Given: 453 grams of oxygen gas O 2 (g) ◦ Note: atomic mass of oxygen is grams of O 2 are given The atomic mass of O 2 is 32 grams. 453/32 = moles of O 2

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Step 4: Determine amount of moles of wanted by mole ratio 2H 2 (g) + 1O 2 (g) 2H 2 O(1) Two moles of water are formed for every 1 mole of oxygen Hence there is a 2:1 ratio of hydrogen to water Hence if you are given moles of O 2 you will form 2 times as much H 2 O. You will form moles of H 2 O

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Step 5: Determine amount of grams are made from the molar mass and desired moles Molar mass of H 2 O(l)? Not needed! You would reversing step 4. SKIP!

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Step 6: Determine amount of molecules of wanted formed using Avogadros number. = 6.02X10 23 molescules/mol Things to remember - 1 mole of anything is 6.02X10 23 units - Hence 1 mole of bananas is 6.02X10 23 bananas - Or 1 moles of water molecules is 6.02X10 23 molecules So there are X 6.02 X molecules of water formed in this reaction X6.02 = X10 23 molecules of water

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Step 7: Write it all out in stoichiometry method and circle answer Correction! 1)Write in scientific notation 2)Consider your number and write your answer in correct significant figures 1)All scientific calculations are based on accuracy. Hence you need to go with the low amount of significant figures. My Calculation: 28.32X6.02 = X10 23 molecules of water 1)Write in scientific notation 1)1.7049X10 25 molecules of water 2)Consider your number and write your answer in correct significant figures 1)My Calculation: (4 sig figs) X6.02 (3 sig figs) = X10 25 molecules of water 2)Final answer should have 3 significant figures 1)1.70X10 25 molecules of water

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