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Real-World Applications of Logarithmic Functions

Before beginning this tutorial, you may wish to review the tutorial on Arithmetic With Numbers In Scientific Notation.

Example 1

pH

In chemistry, a solution’s pH is defined by the logarithmic equation , where t is the hydronium ion concentration in moles per liter. We usually round pH values to the nearest tenth.

a. Find the pH of a solution with hydronium ion concentration 4.5 x 10-5

b. Find the hydronium ion concentration of pure water, which has a pH of 7.

Solutions

  1. If t=4.5 x 10-5, then p(t)= -log10(4.5 x 10-5)= -(log104.5 + log1010-5)= -(log104.5 + (-5)(log1010))= -(.6532+-5)= -(-4.3468)=4.3.

 

  1. Since water has a pH of 7, we know 7=-log10t  and so 7=log10t-1; thus 107= t-1, and so the hydronium ion concentration of water is t=10-7 moles per liter.

 

Example 2

Measuring decibels of sound

The loudness of sound is measured in units called decibels. These units are measured by first assigning an intensity I0 to a very soft sound (which is called the threshold sound). The sound we wish to measure is assigned an intensity I, and we measure the decibel rating d of this sound with the equation .

  1. Find the decibel rating of a sound with intensity 5000I0.
  1. If a sound has a decibel rating of 85, how much more intense is it than the threshold sound?

 

Solutions

            a. =  decibels
            b.  , where the sound in question is k times as intense as
                  the threshold sound. Thus , and so the sound is
                  times as intense as the threshold sound.