Lemon juice:
pH = 2.4
pH is defined as the concentration of hydrogen ions in chemical solutions. Mathematically, this can be written as:
pH = -Log₁₀ [aH]
Where aH is the activity of hydrogen ions. We can replace the lemon juice pH value in order to obtain aH:
2.4 = -Log₁₀ [aH]
-2.4 = Log₁₀ [aH]
We take the 10 exponential:
10^(-2.4) = aH
aH = 3.98*10⁻³
So, the hydrogen concentration in lemon juice is 3.98*10⁻³ M.
M is the molarity unit (moles/liter).