So, What is Molarity Anyway?

So, What is Molarity Anyway?

Scribe: Grace K.

Date: November 5th, 2013

We all know it's a bummer when you can't make it to Mrs. Friedmann's fifth period class. It is the best class of the day after all! However, don't worry about the content you missed. This blog will get you all caught up on everything you need to know!

Handouts

As usual, Mrs. Friedmann had an assortment of handouts for us to pick up when we walked into class today. These included a packet titled Concentration and Molarity PhET-Chemistry Labs and a notes sheet on how to mix a solution given a value of molarity properly. Both can either be found using the links included here or in the Unit 4 Handouts and Unit 4 Notes, respectively.

Last Night's Homework

Mrs. Friedmann checked in the homework due for today in our journal. This included two molarity worksheets that can either be found by this link or in the Moodle handouts folder and the molarity packet that we began in class yesterday.

At this time, we spent a few minutes reviewing question 21 in the Molarity Packet. An in depth explanation can be found in Mrs. Friedmann's key. The main idea is this: molarity compares moles of solute to liters of SOLUTION. This solution consists of both the solute and the solvent-NOT JUST THE SOLVENT! So, in question 21, the mistake was that the student needed 50mL of solution, not 50 mL of water. We will explore this concept further in the simulation section.

The BIG Idea

The in-class notes today were on the main ideas of molarity. These notes can be found either by this link or in the Unit 4 Moodle notes folder. Here's a video to help you understand this very important topic. 

Mixing Things Up

After we understood, this whole "molarity" thing, we decided to "mix things up a bit". It was at this point we looked at the handout on how to mix solutions given the value of molarity. This can be found either by this link or in the Unit 4 Moodle notes folder.

The handout prompts, "Suppose you work in a chemistry lab. Your boss tells you to make 0.50 liters of a 0.75 M solution of sodium chloride. How would you do it??". Mrs. Friedmann gave us five minutes with a partner to discuss possible solutions and this is what the class came up with:

Class Brainstormed Solution:

Step 1: Find moles of sodium chloride.

Step 2: Convert moles to grams so we can actually measure it in the lab.

Step 3: Add water until you get 0.50 Liters.

As it turns out, we are a very smart class and our solution was very correct! A more in depth answer can be found on the answer key. Some students in the class noted that they used the proportionality approach to compute the calculations. However, Mrs. Friedmann stated that she encourages us to use dimensional analysis, as it will be more useful to us in the long run.

"I won't believe it until I see it!"

To make sure we understood the process explored in the notes above, Mrs. Friedmann performed a simulation for the class. She first put 22 grams of sodium chloride in a small container, by zeroing the container on the scale and measuring the substance accordingly. She filled a volumetric flask (see below) with less than 5oo mL of water and added the 22 grams of sodium chloride. Mrs. Friedmann shook the mixture to allow it to dissolve. She also added water to assist in the dissolving process, being sure she did not add too much water to the mixture as such a mistake is hard to reverse. It is best practice to eye the volumetric flask from the level of the meniscus, to make sure one is as accurate as possible. Right before our eyes, Mrs. Friedmann had created 0.50 liters of a 0.75 M solution of sodium chloride, using our calculation of grams of solute necessary. 

Let Us Not Whine About Wine...

Volumetric flasks happen to be one of Mrs. Friedmann's favorite pieces of equipment. (I smell a potential Christmas present! A bejeweled volumetric flask perhaps? ) Today, we learned that the device is apparently used by some people in their kitchen to decant wine. We also learned a bit on the process of wine oxidation. Volumetric flasks are meant to be created very exactly. Once a potential flask is made, it is filled with highly concentrated water. A special machine laser marks the meniscus of the water at an exact measurement, ensuring accuracy. You learn something new everyday!

Last Night's Molarity Homework

Towards the end of the period, we spent a bit of time discussing the Molarity calculations we performed in our journal. The answer key can be found by this link or in the Unit 4 Moodle keys folder. Here are some ideas that may help you to understand this worksheet.

1c. Remember that molecular compounds don't dissociate. The only way to show solid sugar vs. sugar water is using the aqueous symbol (aq) and the solid symbol (s).

3c. Remember that if given mL of solution, you must convert to L using a conversion factor.

6 and 7. These problems are known as dilution problems. This equation will be very useful in solving problems such as these:

M1 x V1 = M2 x V2

....where M stands for molarity and V stands for volume. 

SIG FIGS: When computing calculations of this liking (given a quantity of solute and solution), use the significant figures from the measurement with the smallest number of significant figures given.

Homework

1) Complete the Concentration and Molarity PhET Simulation Packet (in the Unit 4 Handouts folder).  You will need to click on the link posted in the Unit 4 box to access the simulations...the link will take you to a page with 8 or 9 simulations; you will only need to access the ones called "Concentration" and "Molarity".  Due tomorrow.

2) WebAssign 10.2 - Solubility.  Due tonight by 11:59 pm.

THE NEXT SCRIBE IS CAMERON B.