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:

Since 1 liter equals 1,000 milliliters, we can convert:

2.4 liters = 2.4 × 1,000 mL = 2,400 mL

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

ppb = (0.012 mL) / (2,400 mL) × 10⁹

Calculating this gives:

ppb = (0.012 / 2,400) × 10⁹ = 5,000 ppb

Thus, the concentration of ammonia in the solution is 5,000 parts per billion (ppb).