如何計算溶解度?

i take an example of solubility of water
at a specific temperature, e.g. 20 degree Celcius
dissolve the substance into water
and calculate the number of gram of the substance dissolved in 100g of water
if 8.0g of the substance dissolved
then the solubility is 8g per 100g of water at 20 degree Celcius

but there are something that are soluble in water
then the result is infinite