Date: 04/01/99 at 12:38:46 From: Shekman007 Subject: Biology Logarithms On a practice Sat II for Biology, I saw a problem that read: The Ph of an acidic pond is 5. What is the hydrogen ion concentration (moles per liter)? I did not know how to solve this. In the answer key it said that: Ph = -log (hydrogen ion concentration) The answer was .00001. Thus, 5 = -log (.00001). I do not understand how to get this answer using logarithms. Can you please explain it to me in very simple terms, being that I do not know anything about logarithms?
Date: 04/07/99 at 11:53:03 From: Doctor Nbrooke Subject: Re: Biology Logarithms Hello, Logarithms are just another way of expressing exponents. Here is an entry in the Dr. Math archives to show just how logarithms work: http://mathforum.org/dr.math/problems/charley.html Read the definition of Log by Dr. Ken and then come back here. We'll take the formula that you started with (pH = -log([H+])) and work to the answer (solve for [H+]). pH = -log([H+]) Given. pH = log([H+]^(-1)) Since logarithms are like exponents, when you multiply a log by something, you can just move it to the inside of the log as an exponent. 10^pH = 10^log([H+]^(-1)) Take each side to tenth power. 10^pH = [H+]^(-1) Since "log" is just another notation for "log base 10", when you raise a log to the tenth power, the log cancels out. [H+] = 10^(-pH) Take the reciprocal of both sides. That is the general form. To answer your specific question, 5 = -log([H+]) 5 = log([H+]^(-1)) 10^5 = [H+]^(-1) 10^(-5) = [H+] [H+] = .00001 mol/L - Doctor Nbrooke, The Math Forum http://mathforum.org/dr.math/
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