Here is the full list of all 50 Black Mesa achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Decline G-Man’s offer of employment.
Accept G-Man’s offer of employment.
Carry the Hidden Hat to Endgame.
View the *other* secret ending.
Defeat Nihilanth in under 4 minutes.
Take out all three Healing Crystals in Nihilanth’s Chamber at the same time.
Kill all Protozoans in the Borderworld.
Don't kill any Vortigaunts in the Borderworld.
Complete Interloper without taking any laser damage.
Escape the Gargantua’s Lair without taking any damage from Gargantuas.
Find the snack supply canister.
Kill a Controller without killing any of its mind-controlled Vortigaunt slaves.
Deliver the finishing blow to Gonarch with your Crowbar.
Use Cyanogen to kill Gonarch.
Find each member of the missing research teams.
Don't use the Cyanogen and let the Gonarch live.
Get whacked into oblivion in the ledge fight.
Use the welcome mat.
Skip the opening Xen island with a secret jump.
Wait too long to jump into the Lambda Teleporter.
Escape the Hangar in Surface Tension without alerting the Soldiers.
Kill 10 Enemies with Tripmines.
Assemble all the possible Scientists and Guards at the end of Office Complex.
Cook a Grenade for too long.
Die in the AMS Beam before starting the experiment.
Send the Hidden Hat to Xen.
Drown in Lambda Core's coolant.
Use an entire clip of Gluon ammo in one continuous shot.
Kill 20 enemies with the Hivehand.
Kill an Icthyosaur with either the Tau or Gluon.
Overcharge it anyway.
Conduct all of the questionable research experiments.
Get killed by your own Snarks.
Kill 10 enemies with Snarks.
Kill an Assassin with the .357 Magnum.
Kill 10 Airborne Headcrabs using the Shotgun.
Kill a Barnacle with a Satchel.
Help the Barnacle and Headcrab find true love.
Kill 10 enemies with a reprogrammed Sentry.
Kill a Marine with their own grenade.
Headshot 20 enemies with the .357 magnum.
Get overtaken by a Headcrab's ample anterior.
Kill 5 enemies with one grenade.
Use the alternate path in Office Complex.
Perform surgical caffeine extraction from 20 machines.
Kill a Headcrab using fire.
Refuse to insert the specimen into the Anti-Mass Spectrometer.
Pass the toilet paper to the scientist in need.
Ruin the microwave casserole.
Interfere with the alarm and laptop in Anomalous Materials.