Which elements have the same number of energy shells (levels)? magnesium (Mg)
rubidium (Rb)
boron (B)
iodine (I)
argon (Ar)
silicon (Si)

75 g/dm³

Explanation:

The following data were obtained from the question:

Mass of solute (NaCl) = 15 g

Volume of solution = 200 cm³

Concentration (in g/dm³) =?

Next, we shall convert 200 cm³ to dm³. This can be obtained as follow:

1000 cm³ = 1 dm³

Therefore,

200 cm³ = 200 cm³ × 1 dm³ / 1000 cm³

200 cm³ = 0.2 dm³

Thus, 200 cm³ is equivalent to 0.2 dm³.

Finally, we shall determine the concentration of the solution as follow:

Mass of solute (NaCl) = 15 g

Volume of solution = 0.2 dm³

Concentration (in g/dm³) =?

Concentration (in g/dm³) = mass / volume

Concentration (in g/dm³) = 15 g /0.2 dm³

Concentration (in g/dm³) = 75 g/dm³