1 Day Tucson Recommendation
 

Need some guidance. Only 1 day in Tucson. It will be a day trip from Scottsdale area. 2 things I’d like to see for certain: Mission del Bac and the Barrio Historico section. First are they relatively close to each other? Also, is there an Old Pueblo section? I understand the Barrio Historico is a row of color housing. First time to Tucson and would like a cultural experience and not the up and coming metropolitan environment. Thank you for any and all suggestions. Peace and everything good to you !

The Mission dates to the 1700s, is a must see in Tucson; I'm so glad you're including it. Make sure you visit the very interesting mission museum too. You probably know that it is located on the Tohono O'odham reservation.
The Barrio Historico is basically located downtown. You will take I-10 to I-19 to the Mission. When driving north, I-19 to I-10, you'll take the exit for downtown. Cushing, Stone, 18th St, now occupied by offices, some residences. Most of the barrio was demolished to make way for the Convention Center, a travesty. Since you will be downtown and that area will take a short time to see, you should see some other place. You can go to the Tucson Museum of Art. They have wings for Western Art and Mesoamerican artifacts in a former Territorial era home. There are also several other historic homes near the museum.
You won't be far from the University of Arizona either with it's Center for Creative Photography(Ansel Adams archives can be viewed by appointment) and it's own Museum of Art, a Mineral Museum, the State Museums filled with Native artifacts and the AZ Historical Museum is just off campus.
Or drive up to Sabino Canyon in the Foothills and take a tram ride into the canyon, another treasure.
I am listing other options for you as Barrio Historic will not take much time at all. A minimum three hour round trip drive merits at least one more place to visit while in Tucson.
Without traffic problems, it will take you an hour and a half to drive from Phoenix Airport to Tucson. Watch out for the problematic Phoenix/Scottsdale rush hours.

We saw Mission San Xavier this past Monday. I thought it was good, my wife would rate it higher than good. We had a good docent, and as with all tours that really makes it. Do make sure there is no "special services" going on when you want to go as it is sometimes closed to the public. My favorite thing on our 4 day trip was Arizona Desert Museum(specifically, the Raptor Tour). You can see photos and my trip report, if you just scroll thru a bit.

Good advice above. If it is very hot in Tucson (95+) remember that it is cooler up on Mt. Lemmon. My friend Mac is up in the fire lookout tower on Mt. Lemmon 5 days a week until about October. The Kitt Peak observatory is up there too.

The Mt. Lemmon Sky Center is part of the University of Arizona. Kitt Peak houses many more telescopes and is 40 miles SW of Tucson on the Tohono O'Odham reservation. Both are very worthwhile but wouln't apply to the OP's request to fill some time after a visit to the Mission San Xavier.