Idon't understand principles of constant proportions can someone explain this concept to me and how it referes to salt in seawater.

Answer and Explanation:

The principle of constant proportion also referred to as Forchhammer's principle states that although the salinity of various samples of seawater may vary, the ratio of major salts present in the sea water remains same irrespective of the salinity of different samples of sea water that might undergo variation in their salinity.

He clarified that the saltiness of seawater changes because of expansion of freshwater and not because of that the proportion of significant salts like sodium chloride, potassium nitrate, magnesium sulfate changes.

Here, if the salt mentioned is NaCl , at that point we can get the measure of NaCl just by estimating saltiness and taking its product with the extent/level of NaCl that is consistent for ocean water to get the sum.  

There are various strategies to quantify salinity like by estimating resistivity or the amount of chlorine present in water.  

So, if we need to know the measure of NaCl, we can ascertain by this procedure and here is the importance of Law of constant proportion that as the proportions are equal we can utilize that proportion to figure a specific ion.