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How to determine pH From pKa

In acid-base chemistry, pH measures the acidity or alkalinity of a solution. It is dependent on both the concentration of the solution and the strength of the acid. pKa represents how acidic a given hydrogen atom in a molecule is. The relationship between the two is that the pKa of an acid is the pH at which it is exactly half-dissociated, or in equilibrium. You can calculate the pH if you have both the pKa value and the concentration of the solution.

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  1. Enter the pKa value into the calculator and divide by 2. The equation for calculating pH is that pH is equal to half the pKa minus half the log (to base 10) of the concentration, or pH = 1/2(pKa) - 1/2 (logC) where "C" represents the concentration. For example, if the pKa of the solution is 5.4, divide 5.4 by 2 to give 2.7.

  2. Find the log of the concentration by entering the figure into the calculator and pressing the "Log" key. For example, if the value of the concentration is 0.02 M, enter "0.02" and press "Log" to see the result of -1.69897000433602. Divide this figure by 2, to give -0.84948500216801.

  3. Deduct the result of step 2 from the result of step 1. In the example, step 2 has a negative value, so add it to the value arrived at in step 1. 2.7 - (-0.84948500216801) equals 3.54948500216801. This rounds to 3.55 as the pH value of the solution.

  4. Tip

    A pH unit change of 1 represents a 10x [H+] ion change in concentration.

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Things You'll Need

  • Scientific calculator

About the Author

Isobel Phillips has been writing technical documentation, marketing and educational resources since 1980. She also writes on personal development for the website UnleashYourGrowth. Phillips is a qualified accountant, has lectured in accounting, math, English and information technology and holds a Bachelor of Arts honors degree in English from the University of Leeds.

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