Textbook Question 6B.3

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Bella Wachter 1A
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Textbook Question 6B.3

Postby Bella Wachter 1A » Wed Jan 13, 2021 6:42 pm

I wanted to ask if anyone knew why on this question, the pH answers are rounded to one decimal place only.

In Part A, we are taking the -log of 0.025 (which has 2 significant figures), so the pH answer should be rounded to two decimal places, since only the places after the decimal count towards significant figures in logarithms.

Same thing goes for part B, but the answers on the answer key are only 1.6 and 1.7, which would imply that our original concentrations both had 1 significant figure, which is not the case.

Am I missing something here? Could someone explain?

Actually, to add: several of the book solutions are structured like this, where the number of sig figs in the concentration is the number of sig figs in the pH logarithm-derived answer (which is not what was specified in lecture).

It appears that most of their answers are rounded to the hundredths place, regardless of the number of sig figs in the original concentration value. Is there a reason for this?

Joseph Hsing 2C
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Joined: Wed Sep 30, 2020 9:42 pm

Re: Textbook Question 6B.3

Postby Joseph Hsing 2C » Wed Jan 13, 2021 11:18 pm

I don't know the full context of the question but just from reading what you typed, I'm guessing there's some confusion on sig figs: 0.025, 1.6, and 1.7 all have 2 sig figs. Make sure to include all non-zero digits left of the decimal in your sig fig count.

I hope this helped!

Karl Yost 1L
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Joined: Thu Dec 17, 2020 12:19 am

Re: Textbook Question 6B.3

Postby Karl Yost 1L » Thu Jan 14, 2021 11:17 am

I believe that the textbook is incorrect here, as sig. figs. should = #decimal places.

As a general rule of thumb, we quote pH and pOH values to 2 decimal places.

Jiayi Wu 3J
Posts: 50
Joined: Thu Dec 17, 2020 12:17 am

Re: Textbook Question 6B.3

Postby Jiayi Wu 3J » Thu Jan 14, 2021 11:37 am

Significant figures are different from decimal places. 1.7 and 1.6 both have 2 significant figures. The rules are as follows:
- All non-zero numbers ARE significant.
- Zeros between two non-zero digits ARE significant.
- Leading zeros are NOT significant. (e.g. 0.00078 has 2 significant figures instead of 6)
- Trailing zeros to the right of the decimal ARE significant. (e.g. 0.780 has 3 significant figures, not 2)
- Trailing zeros in a whole number with the decimal shown ARE significant. (e.g. 190.)


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