What is the difference between pH and hydrogen ion concentration?

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Answer 1 · 2014-08-20T22:06:49

The hydrogen ion concentration is the number of moles of $H^{+}$ $"H"^+$ ions in a litre of solution.

The concentration can range from about $10 mol/L$ $"10 mol/L"$ to about $10^{- 15} mol/L$ $10^(-15) "mol/L"$ .

To convert such a wide range of numbers into ones that are easier to visualize, chemists take the negative logarithms of the concentrations.

This gives numbers ranging from about -1 to +15.

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If you know the concentration, you can calculate the pH.

EXAMPLE 1

Find the pH of a solution in which $[H^{+}] = 0.0025 mol/L$ $["H"^+] = "0.0025 mol/L"$ .

Solution

$pH = - log [H^{+}] = - log (0.0025) = 2.60$ $"pH" = -log["H"^+] = -log(0.0025) = 2.60$

If you know the pH, you can calculate the hydrogen ion concentration.

EXAMPLE 2

Calculate the hydrogen ion concentration for a solution with pH 2.30.

Solution

Since $pH = - log [H^{+}]$ $"pH" = -log["H"^+]$ ,

$[H^{+}] = 10^{-pH} mol/L$ $["H"^+] = 10^"-pH" " mol/L"$

So $H^{+}] = 10^{- 2.30} mol/L = 5.0 \times 10^{- 3} mol/L$ $"H"^+] = 10^-2.30 "mol/L" = 5.0 × 10^(-3) "mol/L"$