Significant Figures and pH (pOH as well)


Back to pH discussion

Back to pOH discussion

Return to Acid Base Menu


Here is the example problem: Calculate the pH of a solution where the [H+] is 0.00100 M. (This could also be a pOH problem. The point being made is the same.)

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 is 3.

You would be graded wrong!! Why? Because the pH is not written to reflect the number of significant figures in the concentration.

Notice that there are three sig figs in 0.00100. (Hopefully you remember significant figures, since you probably studied them months ago before getting to acid base stuff. THEY ARE STILL IMPORTANT!)

So, our pH value should also reflect three significant figures.

However, there is a special rule to remember with pH (and pOH) values. The whole number portion DOES NOT COUNT when figuring out how many digits to write down.

Let's phrase that another way: in a pH (and a pOH), the only place where significant figures are contained is in the decimal portion.

So, the correct answer to the above problem is 3.000. Three sig figs and they are all in the decimal portion, NOT (I repeat NOT) in the whole number portion.

Here is an incident I saw on Yahoo Answers. The kid said the pH was 10.7, but got graded with some points off (I don't know how much.) The comment the kid made was that 10.7 was three significant figures.

WRONG!! The 10 does not count. The correct answer would have had three figures in the decimal portion, as in 10.730 or 10.711.

I would have emailed the kid about this, but she had email turned off. Oh well.

In another case, a person emailed me with the same type of complaint. See my desciption of the incident under Problem #2.


Back to pH discussion

Back to pOH discussion

Return to Acid Base Menu