I saw that in the junkyard the other day. I have a 760 turbo so my setup is different.
It definitely won't run with that hose missing. There is an idle control valve below the intake manifold - usually round, silver, about 6" long. It should have two hoses coming from it. If one is missing - that is probably the hose you are looking for.
Note. Onlooking back at the picture - my answer above may not be right, as I can't picture a hose going all the way from the firewall there to under the manifold. You can always post that pix on the Brickboard ..
The Volvo Resource: brickboard.com after you set up an account. Someone will know for sure.
Note on the note. I just noted when you posted this, Hope by now you have found the answer.