OK, so suppose your 100 lbs of weight is from a shelf bracket supporting a shelf 1' wide. Let's take the worst case, that all the weight is at the front of the shelf. Then that's a moment of 100 ft-lbs.
If the shelf bracket has two fasteners into the 2x4 that are 1' apart, then the load on the top fastener is 100 lbs of tension. If you are holding the 2x4 5 ft from the ground and have the bottom end against your foot, you have to apply a tension force of 20 lbs.
Now if the 2x4 is glued to the face of the masonry wall, and is 8' long, then a tension force of 25 lbs at 4' from the bottom gives 100 ft-lbs. If the adhesive is along the entire length of the 2x4, then that's a tension load on the adhesive connection of only 3.125 lbs/ft.
Yours, Wayne