What percentage of pure alcohol is in a shot of 100 proof liquor?

First of all, an ounce of hard liquor (gin, vodka, rum, whiskey, or scotch) contains 64 calories for 80 proof varieties, and 80 calories for those that are100 proof. The proof refers to the percentage of alcohol in the liquor (80 proof is 40 percent alcohol; 100 proof is 50 percent alcohol)