Final answer:
The concentration of lead in the 4-L water sample is 20 ppb. This is calculated by dividing the amount of lead, converted to grams, by the volume of water in liters, and then multiplying by 10^9 to get the concentration in ppb.
Step-by-step explanation:
The concentration of lead in parts per billion (ppb) can be calculated using the mass of the solute and the volume of the solution. Here, we are given an 80 µg (micrograms) sample of lead in 4 liters of water. To find the concentration in ppb, which is parts per billion, we use the formula:
Concentration in ppb = (part × 10⁹) ÷ Total volume in liters
To make the units compatible, since we have micrograms of lead, we convert them to grams by noting that 1 µg is equal to 1 x 10-9 grams. Then, we calculate the concentration:
Concentration in ppb = (80 x 10-9 g) ÷ 4 L = 20 x 10-9 g/L
Therefore, this is equivalent to 20 parts per billion (ppb), so the correct answer is A: 20 ppb.