Significant Figures for Test 1

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Jasmine 2C
Posts: 119
Joined: Wed Sep 18, 2019 12:18 am

Significant Figures for Test 1

Postby Jasmine 2C » Tue Oct 08, 2019 10:45 pm

I was wondering when they (graders) want us to round to sig figs. Because the textbook and the sigfig pdf Dr. Lavelle posted on his website says to only round at the final step. But when I keep all numbers during multiple-step calculations on the textbook problems, my final answer ends up being wrong according to the solutions manual.
For example, E21 part B asks us to calculate the amount in moles and the number of molecule units in 25.92 mg of HF. I calculated the number of moles by dividing 25.92 * 10^-3 g by 20.006 g/mol, which is 1.295611317 x 10^-3 moles. Then I used this number to calculate the amount of molecule units by multiplying that number by 6.022 x 10^23 molecules/mol, which is 7.802171349 x 10^20 molecules. Thus, I rounded this number to 4 sigfigs and got 7.802 x 10^20 molecules.
However, this answer is wrong. The answer they have is 7.83 x 10^20 molecules because they had rounded the 1.295611317 x 10^-3 moles to 1.30 moles already, and used this number to calculate the number of molecules.
If I didn't round it early on, would I get docked points? Or vise versa, if I did round early on, would I get docked points?
I'm so confused!

Abigail Menchaca_1H
Posts: 63
Joined: Sat Sep 07, 2019 12:19 am

Re: Significant Figures for Test 1

Postby Abigail Menchaca_1H » Tue Oct 08, 2019 11:17 pm

I think you should round early on because that l.2595611317x10^-3 is too large and is closer to 1.30x10^-3.

Jamie Lee 1F
Posts: 65
Joined: Fri Aug 09, 2019 12:16 am

Re: Significant Figures for Test 1

Postby Jamie Lee 1F » Tue Oct 08, 2019 11:22 pm

I would round at the end of each "step" if that makes sense. Sometimes a problem has multiple parts to it, like for example, you would have to find moles from grams and use that answer to find a limiting reactant- so you should round after you've completed each step and use those rounded answers to find your final answer.

Emma Popescu 1L
Posts: 70
Joined: Wed Sep 11, 2019 12:16 am

Re: Significant Figures for Test 1

Postby Emma Popescu 1L » Wed Oct 09, 2019 11:02 am

My TA said that they are not very strict regarding significant figures on tests


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