How could the index of refraction of salt water be used to find the concentration of a salt solution?

Answer 1

You need experimental data on refractive index vs. salinity. Such data exists. See
http://www.ee.ucl.ac.uk/~mflanaga/java/Saline.html

for example

You also need an experimental measurement of the actual refractive index. This can be performed with a triangular prism containing the solution.