Is mixing permissible in an association for the development of faith, with adherence to the legitimate hijab, and is it a condition to abandon the study of chemistry for the study of Sharia sciences in order to marry a religious young man who believes there is mixing in it?

Gathering for good and cooperating in da'wah (preaching) is permissible within the bounds of Sharia (definition). However, impermissible mixing between men and women is a source of corruption. If the association is mixed without segregation, then it is not permissible to join it; rather, one must beware of it and advise its members. As for your marriage to the young man, if he is a person of religion and good character, then agree to him after performing Istikhara (prayer for guidance). If your chemistry studies involve mixing (of genders), then his advice to you to abandon them is correct, because abandoning them is obligatory in this situation.