Colligative Properties Lab

Today in Chemistry we did the Colligative Properties Lab. The goal of this lab was to use boiling point elevation data to identify an unknown salt. We began by labeling four 100 mL beakers A,B,C, and D and recording their masses in our super neat data tables. Next we filled each of the beakers about half full with distilled water. We recorded the mass of the beaker and water in our data table. We then placed the beakers full of water on the hot plate and heated them to about 85°C at which point we removed them from the hot plate. We determined the boiling point of the water in beaker A by noting the plateau on our lab pro. We recorded this number and proceeded to add about 5.0 grams of the unknown ionic solid to beaker B. We placed it on the hot plate and recorded its boiling point in our data table. We then added about 10.0 grams of the unknown solid to beaker C. We placed it on the hot plate and recorded its boiling point once again. Finally we added 15.0 grams of the unknown solid to beaker D, placed it on the hot plate, and recorded its boiling point in our data tables.

As a part of the data/calculations we were asked to calculate the molality of each solution and were given the molal boiling point elevation constant: 0.51°C kg/mol. To calculate the molality of each solution, we used the equation ΔT_b = K_b · m · i. To calculate change in temperature, we subtracted the boiling point of beaker A from beaker B, beaker B from beaker C and so on. We were told the ionic sold has 2 ions, so i=2. We plugged these values into the equation and solved for m to get a value for molality.

To calculate the moles of solute in beakers B, C, and D, we used the molality equation which says that molality = mol solute / kg solvent. We have the value for molality and to find the kg of solvent we simply subtracted the mass of the beaker from the mass of the beaker and water and divided the answer by 1000 to obtain the value in kilograms. To find the moles of solute, we multiplied the molality by kg solvent.

To find the molar mass of the solute in beakers B,C, and D we divided the number of grams of solute in each mixture by the moles of solute obtained in the previous step. We added 5.0 grams to beaker B, 10.0 grams to beaker C, and 15.0 grams to beaker D. These values were divided by the moles solute to obtain the molar mass of the solutes. To calculate the average of the molar masses, we just added them together and divided by three.

The conclusion asks you to decide the formula for the unknown solute, so choose the formula with the molar mass closest to your average. The choices are NaCl, KI, NaNO3 or NaBr. Support your claim with evidence and then calculate the percent error:

| actual value – theoretical value | x 100 %
theoretical value

The lab is due Friday and so are all of the worksheets and Webassigns. Study for the Test Friday!

The next scribe will be Emilio I!

Who's Worth More: Your Lil Brother or Lil Wayne?

We began class today by learning about percent composition in our notes packet. A percent composition compares the mass of each element present in 1 mole to the total mass of the compound.

Sample Problem:

Penicillin F has the formula C₁₄H₂₀N₂SO_{4. What is the mass percent of each element?}

To begin this problem, we calculated the total mass of the compound:

14 x 12 + 20 x 1 + 2 x 14 + 32 + 4 x 16 = 312 grams/mole

To determine the mass percent of Carbon, we multiplied the amount of carbon in the compound (which we used to determine the total mass of the compound) by 100 and divided by the total mass of the compound. We repeated this process with hydrogen, nitrogen, sulfur, and oxygen to find the rest of our results. We found the following:

Carbon had a mass percent of 53.8 %

Hydrogen had a mass percent of 6.4 %

Nitrogen had a mass percent of 8.9%

Sulfur had a mass percent of 10.3%

Oxygen had a mass percent of 20.5 %

After going over the notes, Mr. Lieberman introduced us to the 'How Much are You Worth?' project. The project asks us to calculate the cost of ourselves, a friend, family member, or favorite actor, although, of course, we are all priceless! We are to show all work in a factor-lable style, and are required to include a picture of our person. A maximum of 5 extra credit points can be given for creativity in your presentation style. Mr. Lieberman showed us some examples of cool projects that have earned students extra credit in the past, but remember that going above and beyond is not required.

Then we took our review quiz which we will have some time to finish in class tomorrow.

The % Composition Bubble Gum Lab is due tomorrow.

Web Assign is due Monday.

'How Much are You Worth?' project due Monday.

The Unit Exam is Thursday 10/28.

The next scribe is...Korri H!