Would the color of pure iron (III) nitrate be useful in determining the concentration of a iron (III) solution? Why?
How might one determine the concentration of a colorless (already made) solution like sodium hydroxide?
Determining the color of a pure sample of dry iron (III) nitrate powder would not be useful in determining the concentration of an iron (III) solution. For one thing, sometimes the colors of chemicals can change when they are dissolved in solvent. Also, simply looking at colors is not going to be anywhere near accurate enough to measure concentration. You can determine the concentration of a solution of iron (III) nitrate of unknown concentration by measuring its absorbance of light and comparing it to other values of solutions of known concentration. A chemical solution's absorption of light is directly proportional to its concentration. So if you have a couple of solutions of difference concentrations and measured their absorbance, then you could plot a graph and use it to determine the concentration of a solution of unknown concentration based on its absorbance.
For determining the concentration of a colorless solution like NaOH, the best thing would be to measure the pH. From there, you can determine the concentration of OH- and therefore the concentration of NaOH from that one measurement.