Percent Composition, Molecular Formulas, and Empirical Formulas

Today in class, we learned a lot of things. At first, I found the new concepts very easy, but as we continued, they got harder and harder. First, we practiced percent composition. I slightly remembered this from earlier in the school year, and remembered some of the steps as we were doing the questions. Below, I posted some practice questions that I can use while studying for the exam. Next, we learned about Molecular formulas vs. Empirical formulas. Molecular formulas can  be Empirical formulas if they can not be reduced any more. For example, the molecular formula of a compound may be P₄O_{10 but the empirical formula would be} P₂O<sub>5. There are more practice problems below for those formulas and other practice quizzes for the questions we practiced with these formulas in class today. 

Percent Composition Link</sub>
Molecular and Empirical Practice
Calculate empirical formula with percent composition data given

Hydrate Lab

In class today, my partner and I successfully passed the pre-lab quiz for the Hydrate lab. I was really nervous that I wouldn't pass, but was very relieved once Mrs. Frankenberg approved of our answers. After we got our safety goggles, my partner and I picked a station. My partner measured the mass of the test tube and we both recorded it in the data table (picture below). After about 2 cm of CuSO4 hydrate was placed in the test tube, we recorded the mass and heated the blue solution until is turned whiteish-gray, making sure that it did not burn. After that we measured it and heated it again to ensure that we evaporated all the water in the solution. Again, we measured its mass and filled in our data table. Then, Mrs. Frankenberg looked at our data and said it looked good. So, my partner and I cleaned up our lab. However, we made the mistake of cleaning our test tube with the anhydrous with water. It reacted and hardened, making it very hard to clean. Next time, we definitely won't add water and will just throw the ashes in that trash-bin!
Here are a couple pictures of that lab below:

Molar Mass

We learned how to calculate the molar mass of a compound in class today. I found the concept easy to learn. However, I do need to review my polyatomic ions. 

This is one example we did in class. We still need to round to the correct number of significant figures in the end.

Magnesium chloride = MgCl₂ = 1(24.31) + 2(35.45) = 95.21 g/mol

Mrs. Frankenberg also taught us the diatomic ions. They are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. When these elements are sitting alone, you must double their mass from the periodic table to find their molar mass. 

For example, O₂ = 2(16.00) = 32.00 g/mol

Below, I posted a picture of the polyatomic ions that I can use to study and a link to practice calculating the molar mass of compounds before the exam. 

http://crescentok.com/staff/jaskew/isr/tigerchem/naming/a85.htm

molar mass quiz

Chemical Composition Notes

In class today, we took notes over chemical composition. I learned that a mole is a quantity of measurements and I was reminded that a mole is equivalent to 6.02 x 10^23 representative particles. Mrs. Frankenberg showed us how whole numbers tell us molar quantity in an equation. Here is an example that she showed us in class to demonstrate the concept:

2H₂O -----> 2H₂       +         O_{2
2 moles       2 moles           1 mole}

Next, we practiced problems with moles, and she gave us a handy conversion chart to use while we calculate:



Below are examples we did together in class. Note that we still need to round to the correct number of significant numbers in the end.

Here are a couple websites with more practice problems for converting with moles:
Chemteam: grams to moles
Chemteam: moles to grams

Pretest Reaction

In chemistry class today, we took a pretest over our next unit: Chemical Composition. I thought the test was difficult. However, once I learn the concepts in class, I believe that I will have a better understanding of what's on the exam. A couple terms I remember from the test were empirical formulas and finding the percent by mass. I have a couple pictures and links below that help explain these concepts.

http://quantummechanics.mchmultimedia.com/2011/general-chemistry-general-chemistry/004-empirical-formula-in-chemistry/

http://quantummechanics.mchmultimedia.com/2011/general-chemistry-general-chemistry/004-empirical-formula-in-chemistry/

Writing Empirical Formula Practice Problems

Percent Composition by Mass