The weight of 12 fluid ounces depends on the density of the substance. Fluid ounces are a measurement of volume (i.e. how much space something occupies) as opposed to ounces, which are a measurement of how much something weighs. For example: 12 fluid ounces of nonfat milk weighs 12.96 ounces, whereas one fluid ounce of pure water (which has less density than milk) weighs one ounce.
The same 12 fluid ounces of granulated sugar weighs 10.63 ounces, while 12 fluid ounces of roasted coffee beans weigh only 5.41 ounces. The contents of a 12-fluid-ounce can of soda weigh about 7.64 ounces.
A fluid ounce is equal to 2 tablespoons or 0.03 liter. Twelve fluid ounces are therefore equivalent in volume to 24 teaspoons or 0.35 liter.