To determine the concentration of ammonia in an aqueous solution expressed as parts per billion (ppb), we start with the formula for calculating ppb in terms of volume:

ppb = (Volume of solute in mL / Volume of solution in mL) × 10⁹

In this case, we have a 2.4 liter sample of solution containing 0.012 milliliters of ammonia. First, we need to convert the volume of the solution from liters to milliliters:

2.4 liters = 2.4 × 1000 mL = 2400 mL

Now, we can substitute the values into the ppb formula:

ppb = (0.012 mL / 2400 mL) × 10⁹

Calculating this gives:

ppb = (0.012 / 2400) × 10⁹ = 5,000 ppb

Thus, the concentration of ammonia (NH₃) in the solution is 5,000 parts per billion. This indicates a very low concentration of ammonia in the aqueous solution, which is important for understanding its potential effects and applications in various contexts.