The black coffee, with a pH of 5 has a hydrogen ion concentration of 1 x 10^-5 mol/L. The tomato juice, with a pH of 4, has a hydrogen ion concentration of 1 x 10^-4 mol/L.
By taking the difference between the two respective concentrations, we can compute how much greater the concentration of hydrogen ions in the tomato juice is compared to the coffee.
Difference = (1 x 10^-4 mol/L) - (1 x 10^-5 mol/L)
Difference = 9 x 10^-5 mol/L
Thus, the tomato juice has a concentration of hydrogen ions that is 0.00009 mol/L greater than the the concentration of hydrogen ions in the black coffee.
