What mass of water will fill a tank that is 100.0 cm long, 50.0 cm wide, and 30.0 cm high? express the answer in grams.

Since the density of water is 1g/cm^3, The mass of water needed to fill the  tank is 150000 grams 

The volume of the tank is (100cm x 50cm x 30cm)  =  150,000 cm³ .

If you're working with water, then water has just about 1 gram of mass in each  1 cm³ .  So when this tank is full, it would hold about  150,000 grams of mass in the form of water. 

150,000 grams is the same as  150 kilograms.

If you filled the same tank with a different substance, then there would be more or less mass in it.  Different substances have different amounts of mass in each  cm³ .  The number of grams of mass in each  cm³  of the substance is called the density of the substance.  The density of water is very nearly ' 1 '.  Substances with greater density feel heavier when you pick them up, because they have more mass in each unit of volume than other substances have. 

For example, an  8-oz  drinking glass holds about  237  grams of mass in the form of water when it's full, and that weighs about 8.3 ounces.  The density of gold is about 19, so if you could fill up the same drinking glass with gold, it would hold about  4,520  grams of mass in the form of gold, and that would weigh about 10 pounds !