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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. 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? Step 1: Write out the equation Hydrogen (H2) gas reacts with oxygen (O2) gas to form water (H2O) liquid? H2(g) + O2(g) H2O(l) Step 2: Balance equation H2(g) + O2(g) H2O(1) 2H2(g) + O2(g) 2H2O(l) Step 3: Find moles of given Given: 453 grams of oxygen gas O2(g) ◦ Note: atomic mass of oxygen is 16.00 453 grams of O2 are given The atomic mass of O2 is 32 grams. 453/32 = 14.16 moles of O2 Step 4: Determine amount of moles of wanted by mole ratio 2H2(g) + 1O2(g) 2H2O(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 14.16 moles of O2 you will form 2 times as much H2O. You will form 28.32 moles of H2O Step 5: Determine amount of grams are made from the molar mass and desired moles Molar mass of H2O(l)? Not needed! You would reversing step 4. SKIP! Step 6: Determine amount of molecules of wanted formed using Avogadros number. = 6.02X1023 molescules/mol Things to remember - 1 mole of anything is 6.02X1023 units - Hence 1 mole of bananas is 6.02X1023 bananas - Or 1 moles of water molecules is 6.02X1023 molecules So there are 28.32 X 6.02 X 1023 molecules of water formed in this reaction. 28.32X6.02 = 170.49X1023 molecules of water 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 = 170.49X1023 molecules of water 1) Write in scientific notation 1) 1.7049X1025 molecules of water 2) Consider your number and write your answer in correct significant figures 1) My Calculation: 28.32 (4 sig figs) X6.02 (3 sig figs) = 1.7049X1025 molecules of water 2) Final answer should have 3 significant figures 1) 1.70X1025 molecules of water