Science Buddies: "Ask an Expert"

I am having difficulty understanding the difference between relative salt concentration and absolute salt concentration. Do you express relative salt concentration in a percentage? For example the stock solution is composed of 1 cup of salt and 4 cups of water so the relative salt concentration is 20%. Is there a simple formula to figure out the absolute dilution? If you start out with the stock solution previously mentioned and you measure out 3/4 cups of the stock solution and add 3/4 cups of tap water how do you find the absolute salt concentration of this dilution.

Yes, you can express relative salt concetration in a precentage.

Hi,

Taylor Jones is right, you can express the concentration of salt as a percentage, but you can also report is as a concentration. You haven’t had chemistry yet, so I will try to explain.

First, you are probably working with table salt, so you should you describe your sample as sodium chloride, or NaCl, for short. Salt is a general term, which chemists use to describe ionic compounds that contain a cation (or positively charged ion) and an anion (or negatively charged ion), so there are many different salts.

You should remember that scientists always use metric measurements, and it’s best to do this in science fair projects also. To express the concentration of NaCl as a percentage, you will want to divide the weight of NaCl in grams by the total weight of the NaCl plus water. For example, one quarter cup of NaCl weighs about 75 grams. One cup of water is equal to 240 milliliters or 240 grams. 75 grams dissolved in 240 grams of water is 23.8% (75 divided by 75+240) x 100. Can you calculate your other concentrations?

However, chemists usually report concentration as moles per liter rather than percentage. To do this, you need to look up the molecular weight of sodium and chloride on a periodic table. You should have a periodic table somewhere in your current science book, so look for it and see if you can find sodium and chloride. (They are on opposite sides of the chart.) Sodium has a molecular weight of 23 grams per mole and chloride is 35.5 grams per mole, so NaCl is 58.5 grams per mole. Using the same example above, you will find that 75 grams of NaCl is 1.3 Moles, (75 divided by 58.5), so 1.3 Moles in .240 liters is equal to 5.4 Moles/liter. The maximum amount of NaCl that can dissolve in water is 359 grams per liter, or 6.1 moles per liter.

It would be best if you measured your NaCl and water, and made your calculations for each of your samples, but if you've already done the experiment, you can go back and calculate your results per percentage or concentration in moles per liter.

Let us know if you have any other questions about completing your project.


Donna Hardy

Is this particular percentage or mole per liter formula for relative or for absolute concentration? So when I report my results as how much salt does it take to float an egg, what do I need to include as the answer in my report?... AMount of NaCl as in teaspoons, absolute perecentage, etc.

Thanks!

Hi,

You should state the concentrations of the salt solutions as either tsp/cup, percent salt, or moles/L so whoever is looking at the project will be able to compare the concentrations of salt. Since you haven't had chemistry yet, hopefully the science fair judges will not expect results in moles/liter from you. However, if you want to try this, I can try explaining again. It would be better to use percentage of salt (grams salt/millileters water x100) rather than tsp/cup.

Let me know if this still isn't clear to you.

Donna Hardy

I will use percentage of salt.

Also, what would be my independent, dependent and controllable variables?

Thanks!!

Hi,

Your independent variable will be each egg that you test; the dependent variable will be the concentration of salt that floats the egg. You could also measure the density of each egg by weighing it and measuring the volume of salt water displaced by the egg. You can compare results of old and new eggs, or of a fresh egg and the same egg again after it is boiled.

Donna Hardy

I am also doing this project and I just can't seem to figure out a correct hypothesis for this experiment could you please help me out?
Thanks!

I am also doing this experiment, I need to know how to find the absolute concentration. If I could have an answer as soon as possible that would be awesome.