Out of the given options, the equation used to determine the density of a gas is:

Perfect gas equation

Explanation:

• The Arrhenius equation relates the rate constant of a chemical reaction to temperature.
• The Einstein equation describes the relationship between mass, energy, and the speed of light.
• The Faraday equation relates the amount of electric current to the amount of chemical change.

Perfect gas equation (PV = nRT) relates pressure (P), volume (V), number of moles (n), gas constant (R), and temperature (T). By rearranging the equation, you can derive the ideal gas density formula:

density (ρ) = n/V = (P / RT)

Therefore, the perfect gas equation allows you to calculate the density of a gas based on its pressure, temperature, and the universal gas constant