First Step:

You will need to calculate the density of the "ocean water". This is done by gathering the mass and volume of a specific amount of the water. You should collect this data in the lab: Graduated Cylinder Mass in grams, Graduated Cylinder w/ Water Mass in grams, and Volume in mL of "Ocean" Water. The density of the Ocean Water is then calculated as:

Your density is expressed in g/mL.

To match this density you must figure out the mass and volume of the "submarine" you are making. The mass part is easy, you simply load the balloon with metal pellets and weigh them together. This is the mass of your "submarine". The volume is a little bit more difficult since that is what you will be varying to make the densities match.

The balloon is spherical in nature and thus we can use the equation for the volume of a sphere, 4/3pr³, to calculate the density.

Notice that the "r" is in red. This is because r stands for radius and that is what you will be solving for once you plug in all of your experimental numbers.

For Example: If our calculated density of "ocean water" is 1.2 g/mL (or 1.2 g/cm³) and the mass of the balloons and pellets is 241.5 g. Then...

Solve for r and you get 3.66 cm. Since the diameter is twice the radius, this means you will need to blow your balloon up to a diameter or width of 7.3 cm.