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The actual or free H⁺ concentration in the Krebs-Ringer solution is different to what you have calculated earlier in this question. The free H⁺ concentration can be calculated from the pH ( pH = -log₁₀[H⁺] ) which is 7.4 .
Calculate the true H⁺ concentration (in mM)

x 10^-5 mM

If you've forgotten or didnt know: log₁₀ x = y is another way of saying 10^y = x ; e.g., log₁₀ 100 = 2 is the same as 100 = 10²)

Question for further thought

Where have all the H⁺ gone or come from ?