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### Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 1:08 pm
When finding empirical formula/molecular formula or in general for chemistry, does it matter whether we use 15.99 grams/mole or 16.00 grams/mole for the mass of oxygen? I tried doing the same calculations with both numbers and I get a slightly different answer each time. The result is not off by much, but I would like to know which one is preferable so I do not happen to miscalculate anything for future exams.

### Re: Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 1:16 pm
I believe that using 16 grams/mole for the mass of oxygen should be okay as Dr. Lavelle has done this in his example problems for the Audio-Visual Focus Topics as well as during lectures. I feel that using this shouldn’t affect your answer by much and should still allow you to receive full credit but I am not completely sure.

### Re: Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 1:17 pm
I remembered from class that it's better to aim for 3 digits after the decimal point (for significant figures), and it's definitely more precise to use 15.99 g/mol for oxygen. I don't think points will be taken off for this during exams, but I'm not totally certain about how much you can round up.
Hope this helps! :)

### Re: Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 1:19 pm
Typically you use 16 as the mass for oxygen. For this case the difference between 15.999 and 16 is so small it never really matters once you take significant figures into consideration.

### Re: Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 3:30 pm
That doesn't make a big difference. Since these calculations are not being used for lab procedures, it is not important to be 100% precise. When you find the empirical formula, you round ratios as well. For example 5.12 moles of C can just be denoted 5 C in the empirical formula. Therefore, it is ok to use 16 g/mol for the mass of oxygen. Just remember, however, that the mass for one ATOM of oxygen is approximately 16 g/mol but the mass of a MOLECULE of oxygen (as in oxygen gas) is twice that of the atom, so 32 g/mol. Hope this helped! :)

### Re: Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 4:32 pm
I'm sure you can use 16 grams/mol since Dr. Lavelle has used that in lectures. There's not much difference between 15.99 grams/mol and 16 grams/mol so I would go for the 16 since I'm sure rounding is preferred.

### Re: Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 9:31 pm
There are no differences, really. There are many versions of the periodic table; however, it is best to just follow the periodic table in our book, which is 16.00 g/mol, and Professor Lavelle uses that as well. Besides, after we take significant figures into account, the answer will most likely be the same whether you used 15.99 or 16.00.

### Re: Which mass of oxygen should be used?

Posted: Fri Apr 06, 2018 10:58 pm
I’m sure you can use 16.00 for the mass of the oxygen. Either 15.99 or 16.00 are fairly close, but Dr. Lavelle used 16.00 in his example.

### Re: Which mass of oxygen should be used?  [ENDORSED]

Posted: Sat Apr 07, 2018 2:05 pm
The safest thing to do in an exam is to use the value in the periodic table that is provided for you.

### Re: Which mass of oxygen should be used?

Posted: Wed Apr 11, 2018 11:34 pm
I think it is okay to use 16. I usually just use it because it is on the periodic table on the front of the textbook.

### Re: Which mass of oxygen should be used?

Posted: Thu Apr 12, 2018 12:28 pm
Since the last digit in an answer is typically an estimate I don't believe it matters.

### Re: Which mass of oxygen should be used?

Posted: Thu Apr 12, 2018 12:30 pm
I think that using 16 is also fine because it is close enough to the actual value. The TAs have said that they take into consideration rounding when grading the tests and exams.

### Re: Which mass of oxygen should be used?

Posted: Thu Apr 12, 2018 12:33 pm
I think that using 16 is also fine because it is close enough to the actual value. The TAs have said that they take into consideration rounding when grading the tests and exams.

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 5:38 pm
That being said, will utilizing 15.99 grams/mole or 16 grams/mole entail that the TA(s) or test grader understands the differences in sig figs the end answer will hold because of these varying numbers/computations?

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 5:58 pm
I just use 15.999 as I believe it would give a more accurate answer and at the end of all my work I use Sig. Figs or round if the question asks me too.

Best,
Roger Bazan

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 6:29 pm
It really does not matter much for most of the problems we do in this class. It will matter for cases in which we are working with sensitive analysis where numbers are maybe (10+ sig figs) and such differences will start to matter in reaching your conclusions.

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 7:32 pm
I use 16.00

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 7:47 pm
Good practice would be to use 15.999. But in exam conditions, when you might not have enough time to type in a lot of decimal places/digits, I asked my TA and she mentioned that she would not deduct points as long as your working out is correct and showed and also as long as the final answer is very close to the right answer. For example, if the actual answer was 0.998, and you wrote 1.000, then she said she would still accept it.

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 7:51 pm
I typically use 16 just to get an even number of moles if thats what im trying to find

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 9:05 pm
within the text book periodic table and on our first in-discussion test I believe that 16.00 was used so I would stay safe and utilize this number

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 9:08 pm
Dr. Lavelle usually rounds and when I do some practice problems with the actual number, I end up getting the same thing so I don't think you should stress to hard about that.

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 9:11 pm
The periodic table given to us on the test is 15.9994 g/mol. I don't usually round until I get the final answer.

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 9:15 pm
Since the more exact value is 15.9994, 16.00 is actually closer to it than 15.99

### Re: Which mass of oxygen should be used?

Posted: Sun Apr 15, 2018 9:31 pm
There are so many variations for what molar mass to use but for exams Lavelle will provide us with a periodic table so we can just use the mass that is given on the sheet.