precision for molar masses

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Violet Kwan 3H
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precision for molar masses

Postby Violet Kwan 3H » Mon Oct 05, 2020 6:17 pm

When we use the atomic masses of elements for conversions, what decimal place are we supposed to round up to? Or do we not round at all?
For example, on my periodic table, N is listed as 14.007, but the answer book used 14.01. The answers end up being similar, but I am not sure if Dr. Lavelle accepts a specific range on exams due to rounding like this.
Thanks! :)
Last edited by Violet Kwan 3H on Mon Oct 05, 2020 6:49 pm, edited 2 times in total.

Silvi_Lybbert_3A
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Re: precision for molar masses

Postby Silvi_Lybbert_3A » Mon Oct 05, 2020 6:22 pm

I am not sure either if Dr. Lavelle prefers a specific number of significant figures, but I have been familiar with the popular opinion that rounding to the hundredths place (4 significant figures) is sufficient. You may want to keep your rounding consistent through your process.

Margia Adriano 2A
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Re: precision for molar masses

Postby Margia Adriano 2A » Mon Oct 05, 2020 6:31 pm

Another thing that you can do is just avoid rounding until you get to the very end when it's time to officially answer the question and then just go by the significant figures rule or round to the hundredths place by then!

Kevin Pilcher 3J
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Re: precision for molar masses

Postby Kevin Pilcher 3J » Mon Oct 05, 2020 7:48 pm

I always wait to round till the very end and make sure I'm using correct significant figures for my end answer. Dr. Lavelle may have another method but this is what I was taught in high school. Regarding different numbers on the periodic table, I'd use the periodic table in the book considering it is our main textbook.

kateraelDis1L
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Re: precision for molar masses

Postby kateraelDis1L » Mon Oct 05, 2020 8:24 pm

I had the same question! It always sucks to feel uneasy about an answer you just worked so hard for. Usually, I look at the periodic table and round in the beginning (like oxygen is 15.999 but I always round to 16.) When I don't round, my answers tend to have a lot of numbers after the decimal point (which can be kinda annoying to keep writing out and leads to more room for error). When doing the sapling homework, I found that it accepts my answer with no rounding. So that's something to keep in mind so you don't have to keep re doing the sapling problems! lol

Kelly Tran 1J
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Re: precision for molar masses

Postby Kelly Tran 1J » Mon Oct 05, 2020 8:55 pm

Typically you would wait until the very end before rounding to the specified amount of sig figs. If I remember correctly, Dr. Lavelle mentioned that everyone would be using the same periodic table for the midterms and final, but I am not entirely sure.

Megan Singer 3D
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Re: precision for molar masses

Postby Megan Singer 3D » Mon Oct 05, 2020 9:26 pm

Since 14.007 has 5 sig figs, and 14.01 only has 4, if you want to include 5 sig figs in your answer (which you can only do if you know all the other values you're using to 5 sig figs) you have to use 14.007. If you use 14.01 you almost certainly will not get the same answer. However, if you round your answer to 4 sig figs or less you should get the same answer whether you use 14.007 or 14.01. Since in my experience most problems require that the answer is rounded to 4 sig figs or less, I think it will usually be fine to just use a periodic table with 4 sig figs.

Joanne Yuh 3I
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Re: precision for molar masses

Postby Joanne Yuh 3I » Mon Oct 05, 2020 9:48 pm

I would also confirm with Professor Lavelle, but since the problems that require molar masses usually already give a value with significant figures, I don't think it matters that much if you use 14.10 instead of 14.007.

905290504
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Re: precision for molar masses

Postby 905290504 » Wed Oct 07, 2020 3:44 pm

I would say that use the exact molar mass if it is given in the question. I think for exams we'll all use the same periodic tables and we should also then just use the exact molar mass given without rounding. I think a general rule of thumb is always save rounding for the end!

David Liu 1E
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Re: precision for molar masses

Postby David Liu 1E » Wed Oct 07, 2020 4:16 pm

what I did on the homework problem was round the really close ones like oxygen (15.999) and selenium (78.96), but like ones that don't round as well I keep to one decimal point. I've gotten all the right answers with sig figs according to the textbook, so it's what I've been doing since it works.

Brianne Conway 1D
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Re: precision for molar masses

Postby Brianne Conway 1D » Wed Oct 07, 2020 4:22 pm

Kevin Pilcher 3E wrote:I always wait to round till the very end and make sure I'm using correct significant figures for my end answer. Dr. Lavelle may have another method but this is what I was taught in high school. Regarding different numbers on the periodic table, I'd use the periodic table in the book considering it is our main textbook.


Yea this is probably the formal or preferred way to do it, but some teachers/professors don't care too much about an answer being slightly off due to rounding, so I'd suggest asking your TA and hopefully they will know what Dr. Lavelle specifically prefers.

Brennan McGurrr 3C
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Re: precision for molar masses

Postby Brennan McGurrr 3C » Wed Oct 07, 2020 4:25 pm

I asked this question in my discussion section. My TA said to basically follow the values given in the problem. Let's say you were given a value of 4.06 g, then going to 2 decimal places would be fine. But if it was 4.061 g, I would use 3 decimal places.

Mingzi Yang 1E
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Re: precision for molar masses

Postby Mingzi Yang 1E » Wed Oct 07, 2020 10:02 pm

If the molar mass is given in the question, the way I usually would do is not to round it. If the molar mass is not given, then I would write what the molar mass from the periodic table accordingly. But I notice that some molar masses vary (by one decimal, for example) depending on which periodic table I used.

Binyu You
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Re: precision for molar masses

Postby Binyu You » Fri Oct 09, 2020 8:48 am

I believe it should be always rounded to two significant figures after the decimal point. But when molar mass is given, I will probably not round it because accuracy is important.

Griffin G
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Re: precision for molar masses

Postby Griffin G » Fri Oct 09, 2020 8:57 am

Personally, when using set values like molecular weight, I don't round at all until the very end, and then I round to the sigfigs of the experimentally recorded data (like grams of reagent). On exams we will all be given identical periodic tables, and I think it would be best to use the complete values given on said periodic tables and then round at the end like I said.


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