Because the milliliter is a unit of volume, and grams are a unit of mass, the answer to this question depends on what material one is measuring. For one of the most common materials, water, the conversion is simple. Because 1 milliliter of water equals 1 gram of mass, 300 milliliters of water weighs 300 grams.
The answer becomes more complicated if measuring a volume of a less common liquid. For example, the heaviest liquid at room temperature, mercury, has a mass of 13.6 grams per milliliter. In turn, 300 milliliters of mercury has a mass of 4,080 grams, which is a bit more than the weight of two 2-liter bottles of soda.