How many digits to use for molar mass?

[Jessy Ji 2J]

As I was doing the practice problems for the Fundamental section, I found that I am not really sure how many digits after the decimal point should I keep for the molar masses of elements. For example, if the Ag is 107.8682 g/mol. Should I keep all four digits after the decimal point or just two digits is enough (aka 107.87 g/mol)? Also, if I were to keep only two digits after the decimal point, should I do that for all molar masses to be consistent?

[Jack Van Ryan 1A]

Hi Jessy,

It depends on the periodic table you are using. If the periodic table has four decimal places for the silver molar mass, then use four decimal places. However, if the table has two decimal places, then use two decimal places. In short, just go by the periodic table.

Also, I have a quick question. How do you ask questions, because I can't seem to find it anywhere on the website? Is there a button or link in order to ask questions to everyone?

Jack

[Devika Nair 2D]

Hi Jack,

I don't know if there is a simpler way to do this, but first click on "Board index" under "Quick links" at the top of the page. Then scroll down to Chem 14A and choose the forum that you want to post in (right now, we're in "Review of Chemical & Physical Principles"). From there, choose a subforum (e.g. "Significant Figures") and there should be a button to post a "New Topic". Write your subject and question, then post!

Btw, the FAQ at the top of the page has a lot of answers to commonly asked questions about navigating the Chemistry Community website; that's where I found out how to post.

[Chem_Mod]

Hi Jessy,

Definitely go by what the periodic table lists. Try not to round anything until your final answer!

[Desiree Eshraghi 3A]

This very much depends on whether we will have a standard periodic table for our class and to what digit the masses will go to. Different periodic tables have different digit values (though I've usually only worked to the 2nd decimal). I've also had professors in the past who personally do not prefer to what digit so long as the final answer is within an understandable range (with work to support based on a periodic table if needed).

[Allison Peng 1D]

I would say to use 2 digits past the decimal point typically, but when doing the calculation (and utilizing molar mass from the periodic table), the molar mass should not be the factor that is limiting the number of significant figures in the answer. If the other values given in the question (for example, a measurement of mass or volume) goes to 4 or 5 significant figures, then I would typically go one more digit on the molar mass (basically treat molar mass as a constant value, not a measured one, so it shouldn't impact the number of significant figures or accuracy that much).

[Crystal Ma 2J]

I would go by whatever number of digits after the decimal point there are on the periodic table you are using. In your case, I would keep the 4 digits after the decimal point since that is what appears on the periodic table that your are currently using.

[Emily Wu 2G]

Hey Jessy,

I think Dr. Lavelle has a periodic table on his class website and I would use that as the standard periodic table for this class.
For molar mass calculations, I would say keep however much decimal points there is as presented on the periodic table until you find the final answer and then round that to the standard amount of sigfigs to make sure it is as accurate as possible!

Emily