Here's why:

* Ounces (oz) are a unit of weight in the imperial system.

* Grams (g) are a unit of weight in the metric system.

* 1 ounce = 28.3495 grams

To convert 12 ounces to grams, you simply multiply by the conversion factor:

12 oz * 28.3495 g/oz = 340.194 g