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In a quick addition to this post and this post, here’s a note about pH and significant figures. Firstly the ‘rule’.

When taking the log of a number that is expressed in scientific notation (as hydrogen (hydronium) ion and hydroxide ion concentration often will be), the number of significant figures in the mantissa of the number are correctly replicated AFTER the decimal point in the log value.

e.g., in terms of significant figures, in a pH calculation where the [H3O+] = 2.22 x 10-3 M

(i.e., where there are 3 significant figures in the mantissa), then

pH = -log (2.22 x 10-3) = 2.654

(i.e., three significant figures are replicated after the decimal point) If you would like a math explanation of this, see this simple explanation.

Because this is a ‘math thing’, this is actually almost certainly highly irrelevant to the AP chemistry exam, since I never recall a question where the one point for significant figures on the whole exam, was ever applied to a pH calculation, but you never know!