Calculating Molecular Weight

For those of you who enjoyed our Flower Chromatography, we decided on a follow up activity to help us learn more about the dye molecules. We did a Google search to find the molecular formulas of each of the dyes using their trade names like Blue #1, Yellow #5, etc. The result typically showed us a chemical formula and the chemical structure of the molecule. I stopped short of pulling out the organic chemistry model kit (that'll be next!) which is right on a nearby shelf, but I did have the kids try and figure out the molecular weight of each compound.

Hardcore. I know.

But, Dan suggested that we find out how big the molecules are and molecular weight is a great way to quantify that information. We want to see if our observations match what we find out about the size of the molecule.

Here we go.

The first thing I had the kids do was write down the chemical formula for each dye. R11 wrote them down herself and I jotted them down for I9. We pulled up the Web Elements site and we talked about which elements were in our compounds. Also, we looked at all four and observed what elements they have in common and which are not the same. Red #3 is the only dye that had iodine in it. Fancy that.

We also noticed that the dyes are all large carbon molecules.

In order to find molecular weight, you multiply the atomic mass by the number of atoms of that element in the formula for the compound. Once you have the mass of each element represented, you add them all together. So, that covers multiplying and adding decimals.

I do not have huge skills in the area of making printables. I'm more old school than that. We use a lot of loose leaf paper around here. I ought to do a post on that!

We also used a book version of the table- sure would love a big poster!

The kids did the work themselves, but I wanted to give them some structure on how to solve the problem. They are only in 4th and 6th grade after all! I decided making a copy of my first sheet was prudent so both kids could have a worksheet.

As we looked at the Periodic Table of the Elements we talked about the groups of elements and where they are in the table. We talked about how the atoms of particular groups behave and what that means for how they interact with other atoms.

We aren't finished with our calculations yet and we need to check them against the known listed molecular weight. Once we've done that, we can talk about how the dyes behaved compared to each other and if that correlates at all with their size based on molecular weight.

This activity was an experiment based on a fun idea and it turned out to slide right in line with our study of Thomas Edison and our study of decimals. Because, of course, most elements do not have atomic masses that are whole numbers. Sneaky, huh? I love it when a plan comes together!

Stay tuned for more results!