### Molar mass

Posted:

**Thu Oct 11, 2018 1:19 am**How do you know when to multiply the molar mass by the coefficients? Or we do always?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=26&t=33509

Page **1** of **1**

Posted: **Thu Oct 11, 2018 1:19 am**

How do you know when to multiply the molar mass by the coefficients? Or we do always?

Posted: **Thu Oct 11, 2018 3:51 am**

In most of the practice and homework problems it seems that we do take coefficients into account when finding molar mass, but that is not always the case. I cannot give you a for sure rule of thumb, but I will say that there are times when we have not needed to use the coefficients when finding the molar mass.

Posted: **Thu Oct 11, 2018 9:45 am**

You will use only the subscripts in the chemical formula to calculate molar mass rather than using the stoichiometric coefficients that are present when a chemical reaction is balanced. Stoichiometric coefficients are looked at to get the molar ratios of the reagents and products of a chemical reaction and thus used to convert between different compounds through dimensional analysis.

Posted: **Thu Oct 11, 2018 11:13 am**

Hi!

If you're asking when we have a grams--> molar mass--> coefficient, then you only multiply by the coefficient if you're changing compound to another compound. Or if you're changing compound to atom.

In this example, you multiply the given grams by the molar mass to get the moles of CH3OH, and then because you are trying to get to the moles of H2O, you look up at the balanced equation, look for the coefficient of how many moles there are in front of each compound, and multiply by the coefficient 4/2. (2 is on bottom because you want to get rid of the unit of CH3OH.) I hope that makes sense! And that that was your questionXD.

If you're asking when we have a grams--> molar mass--> coefficient, then you only multiply by the coefficient if you're changing compound to another compound. Or if you're changing compound to atom.

In this example, you multiply the given grams by the molar mass to get the moles of CH3OH, and then because you are trying to get to the moles of H2O, you look up at the balanced equation, look for the coefficient of how many moles there are in front of each compound, and multiply by the coefficient 4/2. (2 is on bottom because you want to get rid of the unit of CH3OH.) I hope that makes sense! And that that was your questionXD.