Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
purpose [ˈpəːpəs] n цель f
on purpose наме́ренно
for illustrative purposes в ка́честве иллюстра́ции
for the purposes of this meeting пресле́дуя це́ли да́нного собра́ния
to no purpose напра́сно
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.