How to Get the PH Based on Hydrogen Ion Concentration
When an acid dissociates, or breaks apart, it separates into a negatively charged molecule and a positively charged hydrogen ion. The stronger the acid, the more completely it dissociates and the higher the concentration of hydrogen ions. The pH scale, which measures the acidity of a substance relative to pure water, is based on the negative log of the number of hydrogen ions per mole of acid. Scientists often need to measure an acid's pH to determine if the ion levels are appropriate for a particular purpose, such as neutralizing a base.
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Instructions
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Determine the number of significant digits in the molar concentration. Any non-zero digit as well as any zeros between two non-zero digits are significant. Zeroes at the end of a number are significant if they are behind the decimal point.
For example, 0.010 M has two significant digits.
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Express the concentration in scientific notation.
For instance, 0.010 would become 1.0 * 10^-2.
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Take the negative log of the number. This is the pH of the solution.
For example, -log(1.0 * 10^-2) = 2; therefore the pH of the solution is 2.
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Round your answer to include the same number of digits behind the decimal point as there were significant digits in the concentration.
For instance, because there were two significant digits in the molar concentration, you would express the pH of the solution as 2.00.
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Tips & Warnings
To express a number using scientific notation, place the decimal point behind the first non-zero digit, then multiply this number times a power of 10. The power should indicate the number of places you moved the decimal point to the right (negative power) or to the left (positive power). For instance, the scientific notation for 0.001 is 1 * 10^-3. The scientific notation for 1,000 is 1 * 10^3.
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