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Chem1100a Midterm review up to chapter 2

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Western University
Chemistry 1027A/B
Kay Calvin

Chem1100A T est 1 Review 2012/2013o you by the Science Soph Team General Testing Tips • Know definitions of what something is • What is a node? • What is an electron • Think about what you are given in the question and what you need to find • Use units, write them out • Make sure you finish the question, often a MC answer will be the incomplete step Studying Tips • Read • Don’t look at answer • Write out what you are given • Write out what you need to find • Solve • Check the other answers and write why each is incorrect How to attack the question • The 10 questions you guys got wrong last year Practice Test Questions Question #1 Section from Text • First, we want to write down our known variables. • Density of solution = 1.65 g/mL • Solution = 32.7 % glucose by WEIGHT* (this is key!) • Step 1: Since we are given the percent by weight, we should find the mass of glucose in solution. (Whenever there is no solution VOLUME given, work with the easiest numbers you can. And since the final part of this question involves molarity (mol/Liter), why not just choose 1 Liter? • Solution density: 1.65 g / 0.001 L = x g / 1 L … therefore, x = 1650 g/L Our Solution • Step 2: So our 1 Liter solution weighs 1650 g. 32.7% of that, or 540 grams, is glucose. There are 540 grams of glucose per L of solution. • Molar mass of glucose (C H O ) is 180 g/mol. 6 12 6 • Step 3: Find out how many moles of glucose are in 1 Liter of solution (which is essentially “the molarity of the glucose solution”. • 540 g / x mol = 180 g / 1 mol … therefore x = 3 moles of glucose in 1 L Therefore, the molarity of the solution is 3.00 mol/L (or 3 M). Our Solution Lee’s Solution (found on WebCT) Question #2 You have the pH so you can use that in an equation to get The question the concentration then use that to is asking find mols then you how finally grams which is what you many are looking for. grams of CaCO3 are Keep your work needed to organized so you raise the don’t get mixed up. Remember that you pH from 1.5 are looking for CaCO3 to 2.6 - it is common to convert to grams after you find mols of H+ but that’s not what is being asked! Our Solution Lee’s Solution (found on WebCT) Question #3 • First we need to know that a molecule with a Permanent Dipole Moment: is a molecule differences in electronegativity , resulting in one area attracting more electrons andto becoming negative, and the other becoming positive. • Next, we need to understand the rules for drawing Lewis structures as well as VSEPR theory. Our Solution Rules for Drawing Lewis Structures (Calvin) 1. Calculate total number of valence electrons for your molecule 2. Add/subtract if your molecule is an ion (if it's a cation, you subtract electrons; if it's an anion, you add electrons) 3. Draw the structure with just the central atom having one bond to each surrounding molecule. Subtract 2 times the number of bonds from the total number of electrons 4. Add the remaining electrons as lone pairs around the more electronegative atom (usually the outer atoms) and then less electronegative if you still have some left over 5. Add charges to the atoms, if necessary 6. Add double/triple bonds if necessary (usually if there is a positive charge on one atom and a negative on an adjacent one, it means you can move one of the lone pairs on the negative to form a double bond)** 7. You are done *Make sure to follow the octet rule, cause otherwise it can seriously fuck you up Our Solution • BF3 CF3- Our Solution XeF4 Our Solution • SO3 Our Solution • IF2 Our Solution Lee’s Solution (found on WebCT) Question #4 Our Solution Our Solution Lee’s Solution (found on WebCT) Question #5 solution.tand what is being asked of you – to find the pH of the 2) Even if you don't know where to start, you should look forunits that are common in the different values you are given. To-do list: Density  Mass  Quantity  Concentration  pH So to begin, notice 1.84 g/mL and 10.0 mL, which both have the mL unit. Multiplying these two will give us a number in grams: 1.84 g/mL x 10.0 mL = 18.4 g Our Solution • The question mentions the word mass when they say that the concentrated solution is 95.02 H 4O by mass. And just recently you found the mass of the solution (18.4 g) by multiplying volume and density. So, 18.4 g x 0.95 = 17.48 g Our Solution • Remember that we've been working to find pH this whole time and a number expressed in grams won't do you much- we want to get a H concentration so that we can take the negative log of it to reach our answer in pH. • Finding the quantity is the next step on our “to-do” list: n = m/M = 17.48 g / 98 g/mol = 0.1784 mol Our Solution • You're not quite done yet because although you found the quantity of H 2O ,4it's the H ions that we take the negative log of to find our pH. Simply look at what H 2O w4ll dissociate into to find the proportion of H+ ions that will come out of it: • H SO --> 2 H + SO4 2- 2 4 • THIS IS WHAT MOST PROFESSORS WOULD CALL A
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