## Significant Figures for E.21 (b)

Ashish Verma 2I
Posts: 59
Joined: Fri Sep 28, 2018 12:28 am

### Significant Figures for E.21 (b)

Question E.21 (b) asks you to calculate the moles and number of molecules in 25.92 mg of hydrogen fluoride.

I tried to solve the question but ended up getting an answer with the incorrect number of significant figures and I can't explain why:

$\frac{25.92 mg HF}{1}*\frac{1gHF}{1000mgHF}*\frac{1molHF}{20.008gHF}=1.295*10^{-3}molHF$

Seeing as the initial measured value (25.92 mg HF) has four significant figures, I kept an answer in four significant figures ($1.295*10^{-3}molHF$)

However, the Solutions Manual used the same process but rounded the answer differently, ending up with the answer: $1.30*10^{-3}molHF$

Because of this sig fig discrepancy, my answer for the second part of this question, (the amount of HF molecules in the sample) also varied from the answer in the Solutions Manual, as that answer depended on the amount of moles calculated.

I'm not sure why the textbook answer is only three significant figures when the measured value has four significant figures and I'd appreciate any help! Thank you!

Mhun-Jeong Isaac Lee 1B
Posts: 54
Joined: Fri Sep 28, 2018 12:17 am

### Re: Significant Figures for E.21 (b)

Fascinating. Yes, based off of the work you have shown, it appears to to me that there should indeed be four sig figs. I believe the solutions manual at times chooses to disregard sig figs, so I wouldn't stress about it too much. I hope this response of agreement eases your conscience.

Aakash Tammana 3H
Posts: 18
Joined: Wed Nov 15, 2017 3:02 am

### Re: Significant Figures for E.21 (b)

The solutions manual doesn't always follow the same sig fig notation, and usually when it comes to assignments and tests the work is often as important as the answer. As long as your steps are correct and your answer is on point, you'll be fine.

Joon Chang 2F
Posts: 59
Joined: Fri Sep 28, 2018 12:25 am

### Re: Significant Figures for E.21 (b)

I was confused about this problem as well. I just left the answer with the number of significant figures given in the problem.

405098546
Posts: 32
Joined: Fri Sep 28, 2018 12:24 am

### Re: Significant Figures for E.21 (b)

Ya I was in the same boat. I spent some time trying to figure out why the book had only 3 sig figs. But clearly the answer needs to have 4.