Note! This mission is optional and isn't even displayed in the journal. I'd recommend replaying it from time to time, as you can enrich yourself that way. You will get the chance to receive this mission so sooner than after completing the second main College of Winterhold quest, Under Saarthal, as only afterwards Toldfir will return from the excavations to the College. If you can't find him anyway, complete the third main quest - Hitting the Books.
Find Tolfdir (he's usually at the main square or the Hall of the Elements) and ask if he needs your assistance with anything (screen above). Tolfdir will tell you that he hast lost his alembic and therefore you will be asked to find it.
Tolfdir's Alembic (screen above) is unfortunately hidden in a randomly-selected place in the Hall of Countenance, though you should look for it inside one of the public areas. Therefore you can pass on checking any mage's private quarters. After obtaining the Alembic, return to Tolfdir to return a prize of gold and a random Soul Gem.