If the pH of healthy blood is 7.4, what is the hydrogen ion concentration?

The hydrogen ion concentration is ##4.0 ×10^"-8"color(white)(l) "mol/L"##.

The definition of is

##"pH" = -log["H"_3"O"^+]##

To get the hydrogen ion concentration, we take the antilogarithm of the pH (the ##10^x## button on the calculator).

##["H"_3"O"^"+"] = 10^"-pH"color(white)(l) "mol/L" = 10^"-7.4"color(white)(l) "mol/L"= 4.0 × 10^"-8" color(white)(l)"mol/L"##