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Anne Marie Helmenstine, Ph.D.

Formula Mass Versus Molecular Mass

By March 19, 2012

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Do you know the difference between formula mass and molecular mass?

The formula mass (formula weight) of a molecule is the sum of the atomic weights of the atoms in its empirical formula.

The molecular mass (molecular weight) of a molecule is its average mass as calculated by adding together the atomic weights of the atoms in the molecular formula.

So, since the definitions differ according to whether you are using the empirical formula or molecular formula for a molecule, it's a good idea to understand the distinction between them.

The molecular formula indicates the type and number of atoms in a molecule. The molecular formula of glucose is C6H12O6, which indicates that one molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.

The empirical formula is also known as the simplest formula. It is used to indicate the mole ratio of elements present in a compound. The empirical formula of glucose would be CH2O.

The formula mass and molecular mass of water (H2O) are one and the same, while the formula and molecular mass of glucose are different from each other.


May 7, 2009 at 12:13 am
(1) m.yaseen mowlana says:

very good explanation

June 3, 2009 at 4:07 am
(2) Rubia says:

wao its fantastic determination i learned easily

June 5, 2009 at 9:18 am
(3) anny says:

gud xplanation…..
means molecular mass applys only on disreate molecules

September 7, 2009 at 8:26 pm
(4) F. Amado says:

Thanks for the explanation. Very helpful and in the simplest terms.

September 25, 2009 at 12:36 pm
(5) swati says:

thank you!!!!!!!

February 16, 2010 at 2:23 am
(6) Analida says:

perfect explaintion

April 24, 2010 at 12:11 pm
(7) amin says:

wow excellence way for understanding

May 15, 2010 at 9:53 am
(8) Steef says:

What would you use the empirical formula for?

May 15, 2010 at 10:36 am
(9) MisterG says:

“Empirical” means “by experiment”. A chemical analysis will give the empirical formula (ie the ratio of the elements). You would then use the mass, found by mass spec for instance, to help turn this into the molecular mass.

May 17, 2010 at 11:30 am
(10) Qudratullah says:

Molecular mass corresponds to a molecule, what would a formula mass correspond to?

May 17, 2010 at 5:02 pm
(11) Doug says:

Thank Heavens for our very best.

September 4, 2010 at 12:46 am
(12) payal says:

It’s a perfect

September 13, 2010 at 4:02 am
(13) francis says:

how to get formula mass?
empirical formula
molecular formula

November 1, 2010 at 8:11 am
(14) thomas williams says:

I thought that molecular mass was used for molecules and formula mass was used for ionic substances?

I believe that when you are comparing an empirical formula to a molecular formula you would use the terms “empirical mass” vs “formula mass”.

For Ionic compounds (those containing elements with large differences in electronegativity) one would add up the masses of all atoms present and say that the total was the formula mass.

For Molecules (those containing elements with little difference between their electronegativities – or a compound composed of two nonmetals) one would do the same thing… add up the masses of all atoms present and say that the total was the molecular mass.

As can be seen the process is the same for both. It wouldn’t be proper though to call the total mass of sodium chloride a molecular mass because NaCl isn’t a molecule. One would say: “formula mass” instead.

Thats my take on it. I believe the good doctor was thinking about empirical mass vs. molecular mass. Please let me know if I am wrong.

November 1, 2010 at 8:25 am
(15) thomas williams says:

in my second paragraph above I meant to say: “empirical formula vs. molecular formula”.

January 31, 2011 at 5:47 am
(16) ALLEN says:

thanks !

February 24, 2011 at 4:03 am
(17) @t says:

thankxx i was really worried cauz of my chemistry paper im glad i understand now..:)

March 8, 2011 at 3:43 pm
(18) Deb B says:

Thomas… you’re wrong and you’re not Formula mass refers to a single unit of any formula (ionic or covalent) and is measured just like all atoms are, in amu (atomic mass units).

I think what folks are doing is comparing the wrong two terms. Formula mass vs. Molar mass are better terms to compare, just as empirical formulas and molecular formulas are good comparisons.

Comparing formula mass to molar mass shows equivalent values HOWEVER, the formula mass is for one water molecule or one unit of the NaCl compound. Neither are truly measurable by themselves so we refer to the mass of the “formula”using amu’s. Molar mass however refers to the mass of one mole of ANY compound (ionic or covalent).

Empirical formulas refer to the smallest whole number ratio that represents the number of each type of atom in a chemical compound (again ionic or covalent). Molecular formula refers to the actual (not necessarily smallest whole number ratio) of a covalent compound. For example many sugars have the same Empirical formula of CH2O. However glucose has the molecular formula of C6H12O6. Same ratio just very specific for glucose.

March 24, 2011 at 8:39 pm
(19) Tuan Vo says:

“The formula mass (formula weight) of a molecule is the sum of the atomic weights of the atoms in its empirical formula. ”
I believe that is wrong. the formula mass would apply to any formula not just empirical. Empirical means lowest ratio.
For example you have to find the formula mass for glucose, C6H12O6. The empirical formula would be CH2O and the two formula mass calculations would be different.

June 15, 2011 at 10:13 am
(20) faqir says:

can some one help here please. i have a problem.i want to know actual weight of the porcelain tile. the whole information is following.

the tile size is. 546mm x546mm
the tile thickness is. 11 mm
the tile powder moisture is. 5%
the density is.1650g

July 28, 2011 at 6:30 am
(21) sabyasachisen says:

multiply the tile size with the thickness. you get the volume. you also have the density. you can get the mass of one tile from there. since 5% of the weight is moisture, to get the dry weight, you need to subtract 5% of the the weight from it.

August 7, 2011 at 8:46 am
(22) tanuja says:

thanks! you’re a lifesaver! :)

October 24, 2011 at 11:16 am
(23) junaid says:

very good description….that is the way to teach studends easily …good luck team for future

November 6, 2011 at 12:02 am
(24) julie anna roldan says:

what is formula mass?

November 20, 2011 at 2:49 pm
(25) erhiga ekhama says:

Now i understand it better…………

April 19, 2012 at 4:08 pm
(26) akram baloch says:

Tank you its easy to understand.

April 19, 2012 at 4:11 pm
(27) akram baloch says:

Thank you its easy t understand .

June 9, 2012 at 6:47 am
(28) Keyte says:


June 9, 2012 at 9:59 am
(29) ishaq says:

brilliant explanation.good

June 17, 2012 at 9:58 am
(30) Rajendran says:

Hi, Can you explain me the difference between Formula weight and formula mass ? Are they one and the same ?

June 21, 2012 at 12:10 am
(31) Ashwathi says:

the explanation was quite good !
If the compound does not form discrete molecules, like in the case for ionic compounds, we use formula mass

thank you

July 20, 2012 at 6:48 am
(32) rishu raj says:

thank you for this ,good explanation

August 3, 2012 at 8:53 pm
(33) Narayan Sarkar says:

Thank you…… Go on helping people in this way…….

August 22, 2012 at 3:53 am
(34) hyz canquei says:

hi! can you explain me about the percent composition? .. . i doubt my answers this day ..

August 29, 2012 at 4:16 am
(35) kimbert bendanillo says:

thank you for the explanation…….
i learn more base on this!!!!!!

September 28, 2012 at 5:31 pm
(36) Hal Vandeleigh says:

If I understood this correctly:

Formula mass refers to the molecular weight of the empirical formula. CH3, for example, is an empirical formula as it contains the smallest whole-number ratio possible.

Molecular mass, on the other hand, would be used in reference to something like C2H6, in which he subscripts are NOT in the smallest whole-number ratio.

Both masses are calculated the exact same way but are named differently — the mass for CH3 would be a formula mass while the mass for C2H6 would be a molecular mass.

I assume one reason it’s called the ‘formula’ mass is because the lowest whole number ratio acts like a blueprint for bigger molecular derivations (e.g. NH3, N2H6, and N4H12). Could be coincidence, but I do use it as a device to distinguish the two.

October 30, 2012 at 11:57 am
(37) shah Qasim syed says:

excellent explanation

November 6, 2012 at 3:33 am
(38) Mr.X says:

Thanx very much

November 16, 2012 at 6:29 am
(39) Tanga says:

Yeeeeeee that’s what I like to see

December 20, 2012 at 4:58 am
(40) Leena says:

Both of the definition almost sounds the same

March 9, 2013 at 2:11 am
(41) Micheal deony zulu says:

This is cool,i understood this so easly without complications.

April 4, 2013 at 12:40 pm
(42) sandeep dhillon says:

thanks a lot !!! i think now i understood it clearly..

July 13, 2013 at 5:47 am
(43) Mar Quimpo says:

that explains it…wonderful!!!

September 3, 2013 at 2:07 pm
(44) zamir says:

What is discrete molecule?

September 21, 2013 at 11:49 am
(45) Justine says:

thank you for the very clear explanation. it has helped me a lot.

October 18, 2013 at 7:05 am
(46) dilip says:

The formula mass unit is used to represent the molecular mass of ionic compound and molecular mass for any compound. Therefore, molecular mass can not identify the compound as ionic or covalent but formula mass can do. So. I think the answer is to be rechecked.

October 31, 2013 at 3:46 pm
(47) sana says:

thank. your explanation is simplest and very helpful for young students

January 6, 2014 at 6:57 am
(48) bacolot says:

thx for da explanation

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