Significant Figures for Acids and Bases

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Anusha Sadda Lec4 Dis4F
Posts: 20
Joined: Fri Sep 26, 2014 2:02 pm

Significant Figures for Acids and Bases

Postby Anusha Sadda Lec4 Dis4F » Sat Dec 13, 2014 11:09 am

How are significant figures different when calculating pH or pOH for acids and bases?

HelenMach 1J
Posts: 12
Joined: Fri Sep 26, 2014 2:02 pm

Re: Significant Figures for Acids and Bases

Postby HelenMach 1J » Sat Dec 13, 2014 12:36 pm

The number of significant figures of a pH/pOH is the same as the number of sig figs in a molar concentration.

"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." -ChemTeam


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