To calculate the acid concentration of a sample of acid rain with a pH of 4.20, we need to use the formula for pH, which is given by:
pH = -log[H+]
Where [H+] represents the concentration of hydrogen ions in the solution.
To calculate [H+], we need to take the inverse of the logarithm of the pH value:
[H+] = 10^(-pH)
In this case, the pH value is 4.20. Plugging in the value, we get:
[H+] = 10^(-4.20)
Using a scientific calculator, the result is:
[H+] = 6.31 x 10^(-5) mol/L
Therefore, the acid concentration of the sample of acid rain is 6.31 x 10^(-5) mol/L.