problem with a self join...

I'm a newbie in sql queries and don't know how to solve the following question:

are there subclasses in which ALL chemicals have a MolMass < 400 ?

relation given (prim. key: ChemID):

Chemicals (ChemID, Name, MolMass, SubclassID)


Many thanks

SELECT DISTINCT SubClassID
FROM Chemicals c
WHERE SubClassID NOT IN (
SELECT SubClassID
FROM Chemicals s
WHERE MolMass >= 400
GROUP BY SubClassID)

many thanks for your help !:D