You're asking about the salinity of seawater. Here's how to think about it:

* Seawater isn't a fixed solution: The amount of dissolved substances (salts) in seawater varies depending on location, depth, and other factors.

* Average Salinity: On average, seawater has a salinity of around 35 parts per thousand (ppt). This means for every 1000 grams of seawater, there are approximately 35 grams of dissolved salts.

Therefore, in one thousand grams of average seawater, there are roughly 35 grams of dissolved substances.

Important Note: This is an average. Seawater near rivers or in coastal areas might have significantly lower salinity, while deep ocean waters could have slightly higher salinity.