Science Buddies: "Ask an Expert"

I'm stuck on two of the definitions for my science project, "How Salty Does the Sea have to be?" The two terms are relative salt concentration and absolute salt concentration. I've never learned this before and I can't find the exact definition on Google. Help?!

Maya Bieber,

It would appear that Science Buddies intends for a relative salt concentration to be the amount of salt in any given cup out of the given cup's total amount of matter. This is essentially the percent of salt in the solution. As an example, they give a cup with a half-salt, half-water solution. They state that this is a 50% relative salt concentration.

If I understand correctly, the absolute salt concentration is just supposed to be how much salt you put in a certain cup. So, if we go back to our 50-50 solution example, there would be a one-cup absolute salt concentration if there was also one cup of water in that cup.

It may be that Science Buddies wants you to measure absolute salt concentration in mass because the guide gives the proper conversions for doing so.

I hope this helps. Good luck!

- Ultra

Hi Maya,

I would just like to add to what Ultra said that scientists usually express the absolute concentration of a salt such as sodium chloride in metric units--grams or milligrams for weight and liters or milliliters for volume. Since what you are doing is a science project, you should also use the metric units for calculations.

The absolute concentrations of sodium chloride (NaCl in chemist's shorthand) in your solutions can be expressed in a couple of different ways. In the lab we often weigh out a certain number of grams of a chemical and dissolve it in a specific volume of water, say 100 mL. For example, if I dissolved 20 g of NaCl in a total volume of 100 mL of water I would say that I have a 0.2 g/mL solution of NaCl. This would be an absolute concentration because it tells me exactly how much NaCl is in a particular volume of solution.

The other, more formal way to express absolute concentration is by using molarity. This is a chemists' term that you will learn all about when you take chemistry. Each element has a certain mass that is different from other elements. Sodium is 23 and chlorine is 35.5 so together they make NaCl with a mass (commonly called formula weight) of 58.5. By definition, one 'mole' of a chemical is the formula weight in grams. So one mole of NaCl would be 58.5 g. Again, by chemists' definition, one mole of ANY chemical dissolved in a TOTAL volume of one liter is said to have a concentration of one molar, abbreviated as 1M.

You can convert the amounts of NaCl in your experiments to grams and then express the absolute concentration as either g/mL or moles/L.

I hope this helps without confusing you. Please ask if you have more questions.

Sybee

Learned a lot from the above two replies.