Hi Samrina, it will be depend on what form of hydrocarbons you use. You can better estimate the soluble concentration of single or simple mixture of hydrocarbons rather than applying the crude oil from which the average solubility is less than 2 % (v/v). FRom many leterature you can have some information related with their respective toxicities. Through my experience we can use 2-5 % (v/v) of saturated hydrocarbons, 0.5-2 % for aromatic hydrocarbons and 4-5 %  for crude oil. In average you can work with the effective concentration ranging from 250 -10.000 ppm

I concur with the statements provided by Agung.  The amount will also depend on the size of your treatment system.  Based on your question, you calculated the concentration (mg/L) to add to the solution will add to the pot.  The total mass of hydrocarbon added to each pot will change based on the total volume of solution.  For instance, if you have  a concentration of 50 mg/L and add 500 mL to 'saturate' a small pot experiment, that will be 25 mg of your target applied to the pot per application.  It will be a different mass if you increase or decrease the volume applied.

When you apply the prepared solution into the small pot of soil, if most of the solution is drained away, then the actual amount of the chemicals in the soil might actually much lower than you expect (carried away by the drained water) in the pot. So, pay attention to the water volume you use to prepare the solution.

It depends on type of hydrocarbons you are using. What is the degree of hydrophilicity/hydrophobicity? Can you specify the hydrocarbons you are using. I personally prefer mixing soil uniformly with hydrocarbo(s), which I wish to use. Again concentration will depend on the dgree of toxicity of hydrocarbon(s) to plant system. To begin with, I shall try in the range of 100-1000 ppm and perform further experiments based on the results.