Ask the H225 question again, this time in Mango. A document nests its listings, and each listing nests its hazards, so the condition has to reach two levels down an array. $elemMatch asks whether some element of an array satisfies a selector, which is what "any listing, any hazard" means. Return the name and formula sorted by name; the same seventeen compounds come back, already deduplicated because a document is one compound.
A intermediate exercise, answered in Mango. It is marked by the rows your query returns rather than by how you wrote it, with 3 hints if you want them.
Ask the same question of a chemical dataset in SQL and in Mango and compare the two answers. The database is a SQLite file your browser downloads once and queries locally, so nothing you write ever leaves your machine — which is also why the tool needs JavaScript.