Building Molar Mass

Building Molar Mass, molar mass calculations, mole calculations

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 2nd 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 3rd 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, Cl2, 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.

I have also used the  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 3rd 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 . Just search (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

Concepts: 

Calculating molar mass

Dissociation of ionic salts

Calculating the molar mass of a hydrate

Time required: 

1 class period

Materials: 

periodic table

calculator

Procedure: 

See document and Teachers Guide

Preparation: 

Provide materials for each group.

Attribution: 

Search amazon for unifix cubes or snap cubes

NGSS

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

*More information about all DCI for HS-PS1 can be found at  and further resources at .

Summary:

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Assessment Boundary:

Assessment does not include complex chemical reactions.

Clarification:

Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem- solving techniques.

Students who demonstrate understanding can develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects).

*More information about all DCI for HS-PS3 can be found at 

Summary:

Students who demonstrate understanding can develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects).

Assessment Boundary:
Clarification:

Examples of phenomena at the macroscopic scale could include the conversion of kinetic energy to thermal energy, the energy stored due to position of an object above the earth, and the energy stored between two electrically-charged plates. Examples of models could include diagrams, drawings, descriptions, and computer simulations.

Students who demonstrate understanding can develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction. 

*More information about all DCI for HS-PS3 can be found at 

 

Summary:

Students who demonstrate understanding can develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.

Assessment Boundary:

Assessment is limited to systems containing two objects.

Clarification:

Examples of models could include drawings, diagrams, and texts, such as drawings of what happens when two charges of opposite polarity are near each other.