Science Buddies: "Ask an Expert"

I am trying to help my fifth grade son with this science fair project, but I could use some help! We used 5 cups of water and 1 cup of salt to make the stock solution. Cup 1 held 3/4 c stock solution. We put 3/4 cup tap water in cups 2-5. We measured 3/4 c stock solution and added it to cup 2. Then we took 3/4 c of solution from cup 2 and added it to cup 3. We took 3/4 cup solution from cup 3 and added it to cup 4. We found that the egg floated in cup 2. I am not sure what to do from here. How could I help him with finding the relative salt concentration and explaining what that even means? The example given in the procedure states cup 2 would be 50 % relative salt concentration. How would I find relative salt concentration for cup 1, 3, and 4? (is cup 3 25%, cup 4 12.5%?)
I'm also unsure how to proceed making a new serial dilution. I realize I would start with cup 2, which is half stock and half plain tap. Would I add 1/4 c plain tap water to this cup for a 25% dilution? If so, Would I just add the same amount of tap water to other cups, or get new cups? What would the relative salt concentrations be of each cup in the new serial dilution?

Here is what I got:

stock solution has 5:1 absolute concentration. (5 parts water to 1 part salt = 5:1)

Cup 2:

3/4 cups water + 3/4 cups stock, please take a look at the background tab:

https://www.sciencebuddies.org/science- ... background

new relative concentration = stock / (stock + water) = 1/2


Cup 3

3/4 cups water + 3/4 cups or 1/2 of concentration from Cup 2. Not that the point here is that since you are using equal parts of water and stock solutions, the new relative concentration is 1/2.

Then your fifth grader needs to understand buoyancy. Why do some things float and some sink? how does the salt-water solution(s) affect buoyancy? http://www.britannica.com/science/Archimedes-principle


Then step 9.a asks about smaller steps in concentration. i.e. add less water each time. Say you add 3 parts stock and one part water: new relative concentration is 3/(3+1) or 3/4 or 75% of the old concentration.

Let us know if this is enough to continue. Good luck.

You mentioned that the absolute concentration of the stock was 5:1. I have also seen it written as 1/5 or 20% salt. Can you explain which is correct? Further, how then would I find the absolute concentration of the remaining cups (2-5)? Please respond quickly!
Thanks!