## How much of the atomic mass of an element should be used?

Ranger Saldivar 2D
Posts: 28
Joined: Fri Sep 25, 2015 3:00 am

### How much of the atomic mass of an element should be used?

On page 31 of the course reader I've noticed that when calculating the mole of carbon atoms by dividing the grams of carbon given which was 40.9 g the grams per mole it was divided by was 12.01 instead of 12.011 which is given on the periodic table. Does it matter how much of the atomic mass you use for calculations, of conversions for example, as on the video module of this page Professor Lavelle stated oxygen has 16 grams per mole instead of the 15.999 grams per mole stated on the periodic table. On the video module he stated he used 16 grams per mole when dividing the mass of oxygen given which was 54.5 g and divided by the 16 grams per mole, since 16 has 2 significant figures wouldn't the answer be 3.4 mol instead of 3.41 mol. Thanks for any replies. Ranger Saldivar 3J

Rachel Hunsucker 3L
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

### Re: How much of the atomic mass of an element should be used

Since you are dividing 40.9 g, which has three sig figs, by the mass of Carbon, you would use 12.01 rather than 12.011 because 12.01 also has three sig figs.

DevinSandoval_1D
Posts: 24
Joined: Fri Sep 25, 2015 3:00 am

### Re: How much of the atomic mass of an element should be used

I'm not sure why that is. 12.01 has 4 significant figures whereas the masses of the other elements all contain only three significant figures. If that is the case, then shouldn't 12.011 be rounded to 12.0?

Matthew_2C
Posts: 3
Joined: Fri Sep 25, 2015 3:00 am

### Re: How much of the atomic mass of an element should be used

In your calculations, I would recommend using as many significant figures as you can and only rounding at the end especially with longer problems. In any case I've been told that Lavelle will at most take off 2 points for significant figures, on quizzes at least. However, if anyone knows how Lavelle calculates the answers for the quizzes (using all the digits he can or simplifying as he goes along) that would be more useful. For any practical application, of course, you would want to use as many sig figs as possible in order to be as accurate as possible.

Ranger Saldivar 2D
Posts: 28
Joined: Fri Sep 25, 2015 3:00 am

### Re: How much of the atomic mass of an element should be used

So when using the mass of an electron for like calculating the kinetic energy of an ejected electron do we use the entire mass of 9.10938 x 10^-31 kg instead of rounding it to 9.11 x 10^-31 kg?

Matthew_2C
Posts: 3
Joined: Fri Sep 25, 2015 3:00 am

### Re: How much of the atomic mass of an element should be used

I would yes, and then perhaps round your answer to a smaller number of sig figs if you want. But like I said, for the quizzes it would probably be best to ask Lavelle how HE calculates the questions.