Reading #2 – More in-­‐depth use of balanced chemical equations. Now that we’ve had a basic look at how we can use a chemical equation, it’s time to do some more scientific examples. This practice is called Stoichiometry, the scientific term for calculating amounts of starting or ending materials. We will use the following process to complete these types of examples. The examples will have an equation and a question (challenge posed). 1) Balance the equation. 2) Create a cheat-­‐table using the information given and your periodic table to fill in the masses and moles of product and reactant substances. 3) Calculate answers. This “cheat-­‐table” will use skills you have already mastered in previous lessons, now we combine those skills to answer a bigger question. Example 11: LiOH + HBr → LiBr + H2O :) If you start with 10.0 grams of lithium hydroxide, how many grams of lithium bromide will be produced? Step 1: Balance Equation (I walk you through all of these e xamples in THIS VIDEO if you want to open 2 windows in your screen). This one is already balanced. LiOH + HBr →LiBr + H2O Step 2: Create a cheat-­‐table LiOH + HBr → LiBr + H2O # of moles 1 1 1 1 Molar mass 23.95 g/mol 80.91 g/mol 86.85 g/mol 18.02 g/mol Mass 10 g ? Find this given/wanted Step 3: Calculate Use information from your “cheat table” to calculate your final answer. The trick is to: -­‐ Start with the given information -­‐ Convert to moles if you weren’t given moles (our bridge between substances in the balanced equation) -­‐ Convert from substance given to substance wanted -­‐ If you need to convert to grams again for your final answer, do so. (this should fill the T-­‐chart in a zig-­‐zag pattern. Top, bottom, top, bottom, top). -­‐ The units should cancel each other out so we end up with the units asked for in the question. 10.0 g LiOH 1 mole LiOH 1 mole LiBr 86.85 g LiBr ? grams = 23.95 g LiOH 1 mole LiOH 1 mole LiBr 1 All examples taken from http://misterguch.brinkster.net/WKS001_026_234211.pdf To FINISH calculate the answer (multiply tops, multiply bottoms, then divide your final fraction). !".! ! ! ! ! ! !".!" ! !"#$ !".!" ! ! ! !
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= 36.26 g LiBr Example 2: :) C2H4 + O2 → CO2 + H2O If you start with 45 grams of ethylene (C2H4), how many grams of carbon dioxide will be produced? Step 1: Balance Equation (I walk you through all of these e xamples in THIS VIDEO if you want to open 2 windows in your screen). # of moles 1 3 2 2 O2 → 2 CO2 + 2 H2O C2H4 + 3 Molar mass 28.05 g/mol 31.99 g/mol 44.01 g/mol 18.02 g/mol Step 2: Create a cheat-­‐table Mass 10 g ? Find this given/wanted 1 C2H4 + 3 O2 → 2 CO2 + 2 H2O Step 3: Calculate Use information from your “cheat table” to calculate your final answer. The trick is to: -­‐ Start with the given information -­‐ Convert to moles if you weren’t given moles (our bridge between substances in the balanced equation) -­‐ Convert from substance given to substance wanted -­‐ If you need to convert to grams again for your final answer, do so. (this should fill the T-­‐chart in a zig-­‐zag pattern. Top, bottom, top, bottom, top). -­‐ The units should cancel each other out so we end up with the units asked for in the question. 45 g C2H2 1 mole C2H2 2 mole CO2 44.01 g CO2 ? grams = 28.05 g C2H2 1 mole C2H2 1 mole CO2 To FINISH calculate the answer (multiply tops, multiply bottoms, then divide your final fraction). !"! ! ! ! ! !!.!" ! !"#
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= 141.21 g CO2