One thousand grams of seawater would consist of how many grams of dissolved substances?
The question is wrong in that the answer can only be an average value.
Seawater salinity will vary from place to place and with the temperature of the seawater. Of course the composition of dissolved substances in seawater, along with salt that is, will also vary from place to place.
On average, seawater in the world's oceans has a salinity of approximately 3.5%, or 35 parts per thousand. This means that for every 1 litre (1000 mL) of seawater, there are 35 grams of salts (mostly, but not entirely, sodium chloride) dissolved in it.
Seawater has an average density of 1.027 g/cm3, but this varies with temperature and salinity over a range of about 1.020 to 1.029.
1 cm3 is = 1 ml. Therefore 1000 g of seawater = 973.71 mL.