The plural form of the noun alloy is alloys.
The plural possessive form is alloys'.
Example: We've tested all of these alloys' strength.
The plural possessive of "laboratory" is "laboratories'".
The plural possessive form of "molecules" is "molecules'."
The plural form of the noun synthesis is syntheses.The plural possessive form is syntheses'.
The singular possessive form of synthesis is synthesis's.
The possessive noun for "chemists" is "chemists'."