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Let's discuss significant figures and pH.<br />
Another sample problem: Calculate the pH of a solution in which the [H3O + ] is 1.20 x 10¯3 M.<br />
For the solution, we have:<br />
pH = - log 1.20 x 10¯3<br />
This problem can be done very easily using your calculator. However, be warned about putting<br />
numbers into the calculator.<br />
So you enter (-), log, 1.20, X10 n , (-), 3, enter.<br />
The answer, to the proper number of significant digits is: 2.921.<br />
III. Significant Figures in pH<br />
Here is the example problem: Calculate the pH of a solution where the [H + ] is 0.00100 M. (This could<br />
also be a pOH problem. The point being made is the same.)<br />
OK, you say, that's pretty easy, the answer is 3. After all, 0.00100 is 10¯3 and the negative log of 10¯3<br />
is 3.<br />
You would be graded wrong!! Why? Because the pH is not written to reflect the number of significant<br />
figures in the concentration.<br />
Notice that there are three sig figs in 0.00100. (Hopefully you remember significant figures, since you<br />
probably studied them months ago before getting to acid base stuff. THEY ARE STILL IMPORTANT!)<br />
So, our pH value should also reflect three significant figures.<br />
However, there is a special rule to remember with pH (and pOH) values. The whole number portion<br />
DOES NOT COUNT when figuring out how many digits to write down.<br />
Let's phrase that another way: in a pH (and a pOH), the only place where significant figures are<br />
contained is in the decimal portion.<br />
So, the correct answer to the above problem is 3.000. Three sig figs and they are all in the decimal<br />
portion, NOT (I repeat NOT) in the whole number portion.<br />
Practice Problems<br />
Convert each hydrogen ion concentration into a pH. Identify each as an acidic pH or a basic pH.<br />
1. 0.0015 2.82<br />
2. 5.0 x 10¯9<br />
8.30<br />
3. 1.0 0.00<br />
4. 3.27 x 10¯4<br />
3.845<br />
5. 1.00 x 10¯12<br />
12.00<br />
6. 0.00010<br />
4.00<br />
acid<br />
base<br />
acid<br />
base<br />
acid<br />
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