Salt is a general term. It is the product created when an acid and a base are combined. Salts themselves can be acidic or basic, depending on the properties of their so-called "conjugate acid/base"
I think what you mean is table salt, otherwise known as Sodium Chloride: NaCl
Well, there are two ways at least that I can think of.
1. Take a sample of the water and measure its volume. Then boil away the water and record the mass of whatever salt is left behind. You can then, using proportions, figure out how much salt was in the entire container based on how much salt was in your sample. Then you can divide the mass of all salt by the volume of the water, to get the concentration (measured in the units of mass over the units of volume, like g/mL , g/L, etc)
2. Take a water sample and record its volume and add a compound that will produce an insoluble chloride salt, like Silver. So if you used AgN03 (Silver Nitrate .. be careful with it!). Fill a buret with Silver Nitrate and take a sample of the salt solution. Add Silver Nitrate until the solution stops becoming cloudy. It's very important that you determine exactly how much silver nitrate you need, so that you can calculate the amount of salt.
This way is a bit harder, because you have to use some chemistry. When you find that adding Silver Nitrate no longer causes cloudiness, you can record the mL of Silver Nitrate you used.
The reaction is:
Ag+ + Cl- ---> AgCl(s) (s) means solid.
So the formula to find the mass of it is:
x mL AgNO3 used
---------------- = y
molar mass of AgNO3
To find the molar mass, you look on a periodic table and add the atomic weights of Silver, Nitrogen, and Oxygen (times 3 for oxygen because there are 3 of them in the molecule)
Let whatever that is to be y.
You take this number and multiply it by the molar mass of Sodium chloride, which is the atomic weight of sodium added to the atomic weight of chlorine.
y * (Atomic weight of NaCl) = z g NaCl in sample
This number z is the amount of salt in your sample. You can then use proportions to figure out how much salt is in the entire solution based on how much was in the sample of water you had.
The calculations I have shown you are actually a part of chemistry called stoichiometry. The idea is to convert grams of a compound to moles (a common unit in chemistry. 1 mol is 6.022*10^23 atoms/molecules ... whatever you have)
p.s. If you're pretty sure that the main solute in your water is NaCl, there's another quick way to figure out how much is present.It's based on the idea that in fairly salty water almost all the electrical conductance comes from the dissolved salt. It won't work if the salt level is low. You can make two electrodes out of copper wire, maybe glued to a little slab of wood or something to hold them a fixed distance apart. Then you can dip those in your solution and measure the resistance with an ohmmeter. You may do best to switch the leads back and forth between the wires and measure the average value, to get rid of some problems that arise in dc measurements. If you have access to an ac ohmmeter, it would work better, but they aren't so easy to find. How then do you convert the conductance (the inverse of the resistance) to an estimate of the salt concentration? Simple- you make a series of measured salt solutions, and compare with them.
(republished on 07/13/06)