How much does a true ounce weigh in grams?

31.10 grams
The troy system uses pounds and ounces, so it is similar, but there is one big difference; there an avoirdupois ounce equals 28.35 grams, whereas a troy ounce weighs 31.10 grams. This means one troy ounce is equivalent to approximately 1.09714 avoirdupois ounces.

What weighs 1 ounce exactly?

One-ounce items include a pencil, a slice of whole-grain bread, and a CD. Other small items can be combined to weigh an ounce, which further expands this list.