9.30.13 - Stoichiometry exam and introduction to the Lewis Structure

Stoichiometry exam

In the exam, we covered all of the stoichiometry we've gone over this year so far. This includes converting moles to grams, finding mass given the mass of the reactants, balancing chemical equations, reaction particle diagrams, empirical formulas, and many other application of stoichiometry and the concept of Moles.

My overall score was an 83.3%. I scored 86.7% on the multiple choice, and 80% on the free response. Most of the questions I missed were not because of the lack of comprehension, however I feel I could have done better if I was more comfortable using the stoichiometry and molar mass tools that we learned in class or over the lectures. The hot potato quizzes helped with the test because they gave very similar questions.

Lewis Structures

Starting last Thursday, we were introduced to the Lewis Structure Diagram. This diagram is based on the atoms' outer valance electrons and their ability to bond with other atoms. The core charges of the atoms are equal to the number of dots in the diagrams. Shared electrons in a covalent bond are expressed with a line between the letters.

Some of the rules for Lewis Structures include:

1. The atom has to have the correct letter to represent the element and the correct number of valance electrons.

2. The sum of the shared electrons and the lone pair electrons must be eight--an octet. 

3. Hydrogen is a duet.

The total number of valance electrons in a compound are the valance electrons of the individual elements added up.

F₂=14 valance electrons 

SiH₄= 8 valance electrons 

PCl₃= 26 valance electrons

O₂^-2= 12 valance electrons

We did a Pogil ("Lewis Structure(1)")on this to make sure we understood the material. We also did some whiteboarding to compare our answers with other groups.

Main Ideas

Stoichiometry and the Lewis Structure do not connect in my head yet, because the Lewis Structure is still a new topic to us. However, the main idea of this week was to see if we understood stoichiometry (by taking the test) and becoming familiar with the Lewis Structure. I am relatively comfortable with the new material, so I do not have any questions. I feel that I participated a lot this week in our activities because (I think) I lead a lot of the discussions on the questions in the pogil in our group. My ideas changed about the Lewis Structure because I'm a little more comfortable with it, compared to last year. 

9.23.13 - Reaction Particle Diagram, Stoichiometry (limiting reactants and yield), and Empirical Formulas

Reaction Particle Diagram

The main idea of the Reaction Particle Diagram was to learn the standard way of drawing particle diagrams. When given a reaction, you must...
1. Balance the equation
2. Draw the correct amount of molecules before the reaction (all spread out)
3. Draw the correct number of molecules after the reaction (including the excess molecules)

In class, we white-boarded the packet "Reaction Particle Diagrams" to go along with this idea. This helped us because it gave us a lot of examples and lots of practice to get us comfortable with drawing these diagrams.

Stoichiometry - limiting reactants

The main idea of limiting reactants in stoichiometry was to understand how to calculate mass of a reaction with limited amounts of reactants. To find the mass, set up two (or more) ratios of the given masses of the two (or more) elements to find which one produces the least amount of a substance. Then, once you find the least amount of product that can be made, go backwards to find the amount of other molecules used to make that product.

In class, we worked on a few worksheets (Stoichiometry(6), (7)). The worksheets allowed us to get some more practice with the new idea.

Stoichiometry - yield

The main idea of yield is that never in reaction is there going to 100% of all products as the result of a chemical reaction. There will be some left inside the container, making the amount of solution less than what is theoretically predicted.

We also got a worksheet in class (Stoichiometry 8 - Yield) to get use to doing these types of problems.

Empirical Formulas

Empirical Formulas describes the relative number of each type of atom in a compound. It is given as the smallest whole number ratios. For example, although glucose is C₆H₁₂O₆, its Empirical Formula is CH₂O. To find the empirical formula given the percent composition...

1. Assume there are 100g of the product in total. All percents given become grams.

2. Find how many moles of each molecule there are in the product.

3. Divide each number of moles by the smallest number of moles.

4. Make sure they are all whole number ratios.

We worked on a worksheet to become comfortable with the new material. There were lots of practice to do this.

In addition to this, we were introduced to Empirical Units and Molecular Formulas, and Mass Percent. Empirical Units and Molecular Formulas basically take the Empirical formula and multiplies it by the number of atoms in the compound, given the molecular molar mass of the compound. 

Mass Percent is the percent of a specific element or molecule in a compound.

Main Ideas

All the ideas we learned this week connect because we need to know one concept in order to understand the next one. We are not able to find Empirical formulas without knowing the basics of Stoichiometry. This is the same for reaction particle diagrams, limiting reactants, yield, Empirical Units, molecular formulas, and mass percent. I understand the concepts of reaction particle diagrams, limiting reactants, yield, and empirical units/molecular formulas, but I think I need more practice on mass percent. I don't understand the question when it asks "Find the mass percent of the compound in the sample". If the compound is the sample, wouldn't it just be 100%? I understand the steps you take to find the answer, but I don't understand what the number in the answer represents. I feel that I participated well in the learning process because I helped out with white-boarding the answers to the worksheets. I'm fairly confident in the subject we learned this week. I just need to work on mass percent, and maybe a series of random practice problems to know when to use what method of ratios. Because these ideas are a lot like the concepts we learned last week, I have no change in my ideas. However, I'm wondering what kind of questions will be on the actual AP exam? Will these Stoichiometry problems be on there?

9.16.13 - Stoichiometry and Blue #1 Dye Lab Report

Stoichiometry

The main idea of stoichiometry this week was to understand how to use conversions to figure out a problem containing different units. We have done two worksheets on stoichiometry to further understand and master the concept. For example,

How many grams of gold would be produced from 551g of gold (III) oxide in the reaction 2Au₂O₃ --> 4Au + 3O₂? (molar mass of gold (III) oxide is 441.93 g/mol)

The main idea is to multiply the given value by different forms of 1 in order to get the same results in a different unit. For example, in the equation above, the numerator and the denominator all equal each other (1 mol Au₂O₃ = 441.93g Au₂O₃). That way, we can change the units of the given number without changing the actual value (anything multiplied by one will equal itself).

Lab Report - Blue #1 Dye

This week, we performed a lab with Blue #1 Dye and different kinds of sports drinks. The question to guide our lab was "What is the Relationship Between the Concentration of a Solution and the Amount of Transmitted Light Through the Solution?" and "How many grams of Blue #1 Dye is there in 500mL of Powerade and Gatorade?". 

As we collected data, we found that the absorbance and concentration of Blue #1 Dye had a direct linear relationship. This was related by the equation y=0.1385x-0.0078 (x=absorbance and y=concentration).

The activity that really tied everything that we learned this week was to find the mass (g) of 500mL of Powerade and Gatorade. We applied our knowledge of stoichiometry to our lab to see how much dye was in our drinks (the equation and process are all in the lab notebook). We found out that there is approximately 1.98x10^-4g of Blue 1 Dye in Powerade, while in Gatorade, there were 5.15x10^-5 g.

In addition to the lab, we worked on post lab questions. These questions tested us if we knew our material well. The first question asks us how to find concentration given the concentration and volume of the diluted solution. Here, we used the equation M₁V₁=M₂V₂to find the concentration in uM.

Here is the work:

The concentration of the original stock turns out to be 17.5uM.

Main Ideas

The main ideas this week were to become comfortable with stoichiometry, how labs work in the class, how to use our lab notebooks, and how stoichiometry can be applied in labs.

I'm still uncomfortable using stoichiometry to convert units. I wasn't sure where to start when we white-boarded our problems for the mass of Blue #1 dye. I feel that I participated well by knowing how to use the colorimeters properly and doing the prelab activities before the lab, however I don't think I understood stoichiometry as well as I should have. My ideas have changed about converting units. I had thought they were very easy to do, however after learning stoichiometry, I understood that there were more steps in the conversion than what I had imagined.