### pOH

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Sörenson defined pH as the negative logarithm of the hydrogen ion concentration.

pH = - log [H^{+}]

We can define the pOH in a similar way:

pOH = - log [OH¯]

In words, the pOH is the negative logarithm of the hydroxide ion concentration.

So let's try a simple problem: The [OH¯] in a solution is measured to be 0.0010 M. What is the pOH?

The solution is pretty straightforward. Plug the [OH¯] into the pOH definition:

pOH = - log 0.0010

An alternate way to write this is:

pOH = - log 10¯^{3}

Since the log of 10¯^{3} is -3, we have:

pOH = - (- 3)

Which, of course, is 3.

Let's discuss significant figures and pOH.

Another sample problem: Calculate the pOH of a solution in which the [OH¯] is 4.20 x 10¯^{4} M.

For the solution, we have:

pH = - log 4.20 x 10¯^{4}

This problem can be done very easily using your calculator. However, be warned about putting numbers into the calculator.

So you enter 4.20 x 10¯^{4} into the calculator, press the "log" button (NOT "ln") and then the sign change button (usually labeled with a "+/-").

The answer, to the proper number of significant digits is: 3.377. (I hope you took a look at the significant figures and pH discussion. If not, why don't you go ahead and do that right now. I can wait.)

### Practice Problems

Convert each hydroxide ion concentration into a pOH. Identify each as an acidic pOH or a basic pOH.

1) 0.0045

2) 5.0 x 10¯^{10}

3) 1.0

4) 3.27 x 10¯^{3}

5) 1.00 x 10¯^{12}

6) 0.00010

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Suppose you know the pOH and you want to get to the hydroxide ion concentration ([OH¯])?

Here is the equation for that:

[OH¯] = 10¯^{pOH}

That's right, ten to the minus pOH gets you back to the [OH¯] (called the hydroxide ion concentration).

This is actually pretty easy to do with the calculator. Here's the sample problem: calculate the [OH¯] from a pOH of 3.45.

The calculator technique depends on which type of button you have. Let's assume you have the standard key. It's labed EITHER x^{y} or y^{x}.

1) Enter the number "10" into the calculator.

2) Press the x^{y} (or the other, depending on what you have)

3) Enter 3.45 and make it negative.

4) Press the equals button and the calculator will do its thing.

Some people have a calculator with a key labeled "10^{x}." In that case, enter the 3.45, make it negative, then press the "10^{x}" key. An answer appears!! Just remember to round it to the proper number of significant figures and you're on your way.

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