4. The following element contains 12 protons and a mass number of 24. Which of the

It contains 24 neutrons.

Explanation:

Mass number = Protons + Neutrons

If you were to have 24 neutrons, the mass number would be 36, invalidating the known information. You have 12 protons and 12 neutrons to form a mass number of 24. Remember that your protons are the same thing as the atomic number, so you can make an equation using your known information to derive true statements.

You would use the formula:

n = 

Where m is the mass, and M is molar mass (the periodic table would be used for this)

In this case, we need to rearrange the formula to find m

m = nM
    = 5.40 x 6.941
    = 37.5g

Hope this helped! Ask me if there's any part of the working you don't understand :)