. For each substance, find the pH from the given hydronium ion concentration.
(a) coffee, 1.26×?10?^(-5)
(b) milk, 2.51×?10?^(-7)
(c) lutefisk, 5.01×?10?^(-12)
(see http://www.ndt-ed.org/GeneralResources/SigFigs/SigFigs.htm for a good description of how to determine the significant figures of a logarithm. Thanks to Andre for helping me to fix my knowledge base! I tried to add this note as a comment but it vanished.)
Since the power of ten is not part of the original significant figures, only the fractional part of the logarithm (mantissa) is rounded to 3 significant figures, so the answers should be 4.900, 6.600, and 11.300.
To see why your answers are wrong, find the pH value of the concentration 5.02x10^{-12} and compare to your answer to (c). Based on your rounding you would get the same pH value, which can't be.
Your link does not address logarithms. Please look up "significant figures logarithms" for a detailed explanation.
Comments
See http://www.usca.edu/chemistry/genchem/sigfig.htm; esp. “write the number in scientific notation. If you can/must get rid of the zeroes, then they are NOT significant.”
So the power of 10 part of a number in scientific notation does not contribute to the significant figures of the whole number.
My answers are correct.