Science Buddies: "Ask an Expert"

Need help with calculations for relative salt concentration and absolute salt concentration.

1 cup of salt is about 292 g. and 1 qt. of water is 0.946 liters

1 cup of salt and 1 quart of tap water combined = stock solution

mass=m p=density v=volume
p= m divided by v

new concentration = volume of stock divided by (volume of stock + volume of water)

cup 1 - 3/4 stock solution
cup 2 - 3/4 cup of plain tap water and 3/4 cup of stock solution
cup 3 - 3/4 cup of plain tap water and 3/4 cup of stock solution from cup 2
cup 4 - 3/4 cup pf plain tap water and 3/4 cup of stock solution from cup 3

Hi Rob!
So when we calculate concentration of a solution, it's usually dealing with molarity. Here's a quick link on molarity and hopefully this will help you finish calculating/analyzing your data!
http://www.graphpad.com/quickcalcs/molarityform.cfm
Keep in mind that formula weight would be the atomic mass of your entire solution based upon the masses given in the Periodic Table. If you have any other questions, let us know! Best of luck to you.

1 quart of tap water

You have another unknown coming into your ability to calculate things. Tap water typically has disolved salts and minerals in it. You can either get some data from your local water supply company/municipality on what the typical mineral concentrations are in the water supply or you can rerun your experiments using distilled water. The process of distilling water removes most of the salt and mineral content (typically >99 percent at least for distilled water you can buy at most stores). Beware, most bottled water is NOT distilled water!

Rob,

Craig_Bridge is right, tap water does have dissolved minerals and salts, but generally at very low concentrations. Since you are dealing with a very salty solutions, you could probably discount the minerals in the tap water for your calculations. Even really hard water which has the most dissolved salts and minerals has about 0.005 moles/liter of dissolved minerals. It won't really affect your numbers, especially within experimental error (that means the error that you have while making the experiment...like a couple of grains of salt fall out while you are pouring them into your container or an extra drop of water gets into your solution when you are making your dilutions).

tzforbes