Solubility and Ion Concentration W/S

Below is the solution for #8 of the Common Ion, Precipitate and Concentration worksheet. There will be a quiz over a problem like the ones you performed on this worksheet on Thursday, September 16th, 2010. Beginning tomorrow (Tuesday, 9/14), under Chapter 4 on the AP Chemistry web-page, there will be two more practice problems for you. Practice problem #1 will have a solution posted on TeacherTube and practice problem #2 will have just the answers. The quiz will be going in the homework section of your grade.

Chapter 3 Quiz Results

Today everyone who completed the Chapter 3 Quiz was shown his or her results. Many individuals did quite well. Of the 29 quizzes scored, 6 (21% of tests) received a perfect score of 23 out of 23. On the opposite end of the spectrum, there were way too many scores that were low D's and F's. Of the 29 quizzes scored, 8 (28% of tests) scored 61% or lower. This is not acceptable since this material is mostly review of material from the first year chemistry course taken as a pre-requisite for AP Chemistry.

Below is a summary of the statistics from the quiz. I have broken the values into three categories: (1) Overall results (2) Junior results and (3) Sophomore results. I want to discuss a disturbing trend that I saw, that is why the two classes (years) were separated.

________Overal results__ Junior results __Sophomore results
Average: __77.0% __________62.6% __________88.3%
Median:___ 82.6%_________ 60.9%___________87.0%

As you can see, the juniors in the class (13 of the 29 tests) underperformed by a large margin when compared to the sophomores (16 of the 29 tests). I can propose one reason for the disparityin the scores.

• For most of the sophomores, they took the pre-requisite course last year before entering AP Chemistry this year. For most of the juniors, they are one plus year removed from the pre-requisite course. This is a possible reason for the disparity, but not an excuse. There were a number of juniors that did very well on the test.

If you were an average freshman sitting in 1st year chemistry up at CU Boulder, most likely you would have taken high school chemistry your junior year of high school. Thus, the average freshman enrolled in first year chemistry at CU Boulder is also two years removed from the first time he or she was first introduced to chemistry. You may have to work a little harder at the review of the material because it is not fresh in your memory. You at one point were proficient at this material because you earned a good enough grade to take this course. You have shown in the past that you have the ability, you just need to sharpen those skills.

No matter how you did on the first assessment of the year, I would like you think about the following points and did you attempt the following during the chapter to be successful.

• Did you read the chapter, even though you were not specifically told to do so? This is a trait that is incredibly necessary for learning high level (college level) material like you are doing now.
• Did you download the PowerPoint file associated with Chapter 3 from the AP Chemistry calender associated with the AP Chemistry webpage? The PowerPoint is a good summary of the material covered in the chapter and offers good examples.
• Did you consistenly work on WebAssign throughout the chapter? The due date for WebAssign work is always one day after the unit assessment, but that does not mean that you should wait until the very end to do the homework. Your homework is your studying, and waiting till the very end to do the work to test proficiency is not productive at all. Worse yet, doing the majority of the work after the exam is really of no point, yet I had reports of students doing this.
• Did you look at my webpage and go to the Chapters section to view the Vodcasts made for the chapter? This could be of some help because both Vodcasts for the chapter covered major topics covered by the quiz.
• Did you go to the AP Chemistry calendar to download the solutions to the old AP Chemistry problems used as review for the quiz. We did go over the solutions in class, but did you study the problems and solutions outside of class?
• DID YOU SEEK OUT EXTRA HELP IF YOU DID NOT UNDERSTAND A CONCEPT? This is the most important and most effective intervention for yourself. It is your responsibility to insure that you know and understand the material. I will give you opportunities to assess you level of comprehension (homework and quizzes), but you need to seek out help if you do not understand the material. Coming in to see me on an off hour is not your only means of getting help. You can email me a question. If I get enough questions on the same subject or problem, I can address it in class for everyone or address it through a Vodcast. If it is a question specific only to you, I can email you back a response. I gave everyone my cell phone number and have had students text me a question. These are all means of getting help for yourself.

Since this was the first assessment of the semester and some of you were maybe taken by surprise in terms of the depth of the material covered and the fast pace of the course, I am going to offer you the opportunity to earn back one half (1/2) of the points that you missed on the quiz. To do so, you must complete a CHAPTER 3 REMEDIAL ASSIGNMENT on WebAssign. This will be due at midnight, Tuesday, September 7th, 2010. Second, from 2:25 pm to 3:15 pm, Tuesday, September 7th, 2010, you must attend a remedial session in room #327 to go over two new AP Chemistry free response questions. Therefore, if you need to make arrangements for a ride from school or make plans with a coach or sponsor to be late for practice or an event, please make the arrangement. This is a one time deal and no exceptions will be made.

Review for the Chapter 3 Exam

On the events calendar for August 31st, I have placed the file (Power Point) for the solutions to the three old AP Chemistry problems that we have been using as our prime review for the Chapter 3 exam that you will take on Thursday, September 2nd, 2010. Some of the solutions are different than what we performed in class, but that is o.k. There is usually more than one way to solve a problem. Remember, tomorrow, we will be in the computer lab in the library. You can use the time to review and work on WebAssign. Have a good night.

Schedule for the week of 8/30-9/3

I just want to give you some insight to the schedule for the week. Tomorrow (Tuesday, 8/31) we will work on the review packet in class of old AP problems. This will be your review for the exam, which will be all written and consist of old AP problems. You may be given parts to the question on the exam that we have not covered yet. I will supply any information to those parts of the test that you will need to use later in the problem. This is just like the review that I gave you. Wednesday is a late start day, so I am going to give you the time to work in class. I will try and supply computers so you can log onto WebAssign and work on your homework. Thursday will be the exam. I hope to have the tests graded by Friday and we can go over them. I will make the Chapter 3 WebAssign due on Saturday, Sept. 4th to ensure that you have the full day after the exam to finish any remaining problems.

GRADES HAVE BEEN UPDATED!!!

I have just updated grades. Some individuals are missing minor assignments (like the signed Lab Safety Contract and Class Policies and Procedures), but because there are very few grades to buffer a bad grade, those individuals have very poor (I'm talkin' "F"!!) grades. Please look at your grade and get the required material to me. The Lab Safety Contracts and Class Policies and Procedures can be found on my teacher pages. I think that activities start taking grade reports this Wednesday for eligibility, so the sooner the better for rectifying the situation.

Copper (?) Iodide Lab Write-Up Due Tomorrow & Preparation

Remember, that tomorrow your lab first write-up for the year is due. Just as we discussed in class, you are not only doing the write-up for a grade in may class, but you are also auditioning for a college or university to receive lab credit if you end up passing the AP Chemistry test next spring. My big area of emphasis on the lab report will be the SEE's and the conclusion. Please make sure that your SEE's address situations that are plausible to your results. If you are overly generic, be prepared to receive no points for your SEE's portion of the report. If you are very specific in your plausible explanation, but contradict your findings, you will receive some credit. So it is better to be intricate and misguided than generic and not address anything of substance. The conclusion must address if the theory was met. Remember, you only have two choices for a plausible formula for copper (I or II) and iodide. Please also include a statement on how you would improve the lab. Just like any other report for any other class, you may want someone to read it for you and critique your work.

In the Chapters portion of my web page, I have posted two links for Vodcasts (new term, catchy, isn't it??) One is for empirical formula determination through combustion analysis and the other is how to determine a limiting and excess reactant. Before tomorrow, (Monday, 8/30), please watch the video on how to determine limiting and excess reactants as a refresher. I will cover the material quite quickly, so the Vodcast will allow you to review / learn at your own pace.

Finally, a AP Chemistry student emailed me with a helpful hint for WebAssign and entering scientific notation numbers. If for some reason (this is a random example) you needed to type Avogadro's number into WebAssign, instead of 6.02 x 10^23, you need to type it in just like it would appear as an answer on your graphing calculator: 6.02E23. If you had a small decimal number, you would type (example): 3.87E-7. Remember to keep plugging away at the WebAssign work. We will probably have a test later this week, so the work will probably need to be done by Friday. Also, your will be getting a review packet tomorrow (Monday, 8/30) in class and I will require the answers to be reported on WebAssign PRIOR TO THE CHAPTER 3 EXAM.

Empirical Formula Determination Through Combustion Reactions and Extra Credit Opportunity

Please complete the problem shown below. Have the solution ready to submit at the beginning of class on Friday, August 26th, 2010. To save time and effort on the problem, notice that the measured values are reported in milligrams. Therefore, feel free to work in the unit of millimole. The ratios all remain the same.

Also, if you would like 10 points extra credit towards your homework grade, bring in a 4-pack of AAA batteries. I will stockpile the batteries and if you need batteries for your calculator at any point this year, you can take a pack from the stockpile.

At the end of class today, there was a queston of what to do with decimals (fractions) once the moles of each element in the compound have been divided by the lowest mole value when determining an empirical formula. If a value is rougly nine tenths (X.9) or one tenth (X.1), then feel free to round up (X.9) or round down (X.1). For other decimals or fractions, use the examples below for how to convert the fraction to a whole number. Remember, elements combine in small whole numbers. **The use of "X" "Y" and "Z" are generic to represent any element.

• 1/2 --> (X .5)x2 --> {Example (Y2Z3.5 )2 --> Y4Z7}
• 1/3 --> (X .33)x3 --> {Example (Y1.33Z1)3 --> Y4Z3}
• 1/4 --> (X .25)x4 --> {Example (Y1.25Z1)4 --> Y5Z4}
• 1/5 --> (X .2)x5 --> {Example (Y1Z3.2)5 --> Y5Z16}
• 2/3 --> (X .67)x3 --> {Example (Y1Z2.67)3 --> Y3Z8}

The fractions represented above are the most common (if not only) fractions you will encounter when doing an empirical formula determination.