17.5 g
35 ppt stands for 35 parts per thousand. So let's convert that to a decimal number by taking 35 and dividing by 1000.
35/1000 = 0.035
Now multiply that number by the number of grams of seawater you have. So
0.035 * 500 g = 17.5 g
So you have 17.5 grams of salt when you have 500 grams of seawater.
