Building Molar Mass

An advantage to teaching on the trimester schedule allows me the opportunity to teach the same course again roughly twelve weeks later. So after grading my 2^nd trimester students’ Chemistry B final exams, I was able to evaluate certain topics that caused my students problems, reflect on my teaching, and then determine how I was going to better prepare my students in the 3^rd trimester chemistry B class. 

One topic in particular that caused some problems for my students was the dissolving of ionic salts into their appropriate ions and then representing them correctly regarding the appropriate numbers of ions. Having students mention to me that table salt when dissolved in water produces sodium and chlorine and not their respected ions, has always bugged me. Let alone even after correcting students of this common misconception, asking students what is produced when MgCl₂ is dissolved in water often students give me chlorine, Cl_2, as a response due to seeing Cl₂ in the formula and often times when I ask students the reason for their answer they tell me because chlorine is a diatomic molecule. 

Another topic that caused my students some problems was that dreaded dot that represents the bond within the hydrate formula such as CuSO₄Ÿ5H₂O. As much as I stressed that the dot was not a multiplication sign, still a percentage of my students missed that topic in some way or another. Regardless of doing some worksheet exercises beforehand, when my students performed the hydrate lab and then answered questions regarding molar mass of the hydrate or the percentage of water in the hydrate then I would get some really strange numbers. 

So how did I change? Well, my original teaching method consisted of supplementing my notes with technology to show my students the dissolving of ions in solution. The dissolving of table salt for example, has been represented in several YouTube videos.

NaCl dissolving #1: http://bit.ly/1nHiu3J

NaCl dissolving #2: http://bit.ly/1BNjKGG

I have also used the PhET Simulation Salts and Solubility to show what happens when salts dissolve in solution and to focus on the numbers of ions that can dissolve in solution that are more than just a 1:1 ratio.  Next, I used notes and practice exercises to calculate everything involved with hydrates. However this method proved to be ineffective.

            So with the start of the 3^rd trimester, I decided that rather than use the technology to show students things such as molar mass calculations and the dissolving of ionic salts then I would ask them to simply build the salts using colored unifix cubes. Just search amazon for unifix cubes (or snap cubes also look popular).  I have a box of 1000 that I distribute amongst my lab tables.  Well, I don’t know if it was the start of a new trimester of just the capability to literally build, but my students attacked my handout building ionic salts and hydrates and explaining how same colors were same atoms and subscripts outside of the parenthesis were shown by replicating a certain pattern of blocks a certain way.  It was amazing to watch and I began to see understanding. I added teacher checkpoints onto my worksheet to check for this. Overall the activity was a big success and I will definitely continue this activity again using the unifix cubes. I gave an assessment and was pleased with the overall average of my classes seeing a bit of an improvement overall. I have included a link to the document. If you have any questions or comments, please don’t hesitate to contact me. If I missed anything or if you can think of another way to improve this then please share. 

Thanks,

Doug