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.
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means molecular mass applys only on disreate molecules
Thanks for the explanation. Very helpful and in the simplest terms.
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What would you use the empirical formula for?
@Steef
“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.
Molecular mass corresponds to a molecule, what would a formula mass correspond to?
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explanation
how to get formula mass?
empirical formula
molecular formula
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.
in my second paragraph above I meant to say: “empirical formula vs. molecular formula”.
thanks !
thankxx i was really worried cauz of my chemistry paper im glad i understand now..:)
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.
“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.
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
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.
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very good description….that is the way to teach studends easily …good luck team for future
what is formula mass?
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Now i understand it better…………
Tank you its easy to understand.
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Hi, Can you explain me the difference between Formula weight and formula mass ? Are they one and the same ?
the explanation was quite good !
If the compound does not form discrete molecules, like in the case for ionic compounds, we use formula mass
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thank you for this ,good explanation
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hi! can you explain me about the percent composition? .. . i doubt my answers this day ..
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i learn more base on this!!!!!!
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.
excellent explanation
Thanx very much
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Both of the definition almost sounds the same
This is cool,i understood this so easly without complications.
thanks a lot !!! i think now i understood it clearly..