Answer:
0.001 mol/dm^3.
Step-by-step explanation:
The pH of a solution is defined as the negative base-10 logarithm of the concentration of hydrogen ions (H+) in moles per liter (M).
The pH of 3 moldm^-3 means the concentration of hydrogen ions in the solution is:
[H+] = 10^(-pH) mol/L
[H+] = 10^(-3) mol/dm^3
[H+] = 0.001 mol/dm^3
Therefore, the concentration of H+ in pH 3 moldm^-3 is 0.001 mol/dm^3.