# Formula weight and molar mass relationship

### Molecular mass - Wikipedia Molar mass and molecular weight are often confused, but their values are very different. Molar mass is the mass of one mole of a substance. A tutorial on Molar mass calculations with examples and a molecular weight and many other formulas in stoichiometry (the quantitative relationships between . Molecular Weight is the weight given in amu of an atom or molecule. significance of this number is seen in the relationship between the masses of individual.

I think you can see, it's a little bit small probably, on your screen, it says, "1.

## Molar mass

This right over here is the atomic weight of hydrogen. This is, if you take the various isotopes of hydrogen on the planet earth and you take the weighted average. So the ones that are more common, they are weighted higher. The weighted average of their atomic masses is 1.

### Molar mass - Wikipedia

Now for the sake of this first pass, we're just going to say, "It's roughly one atomic mass unit. This isn't precise and it's not how things are defined, but it is a useful way to think about atomic mass or mass on the atomic scale.

Is that one neutron, or one proton have an atomic mass of approximately one atomic mass unit. In other videos we'll talk about why it's not defined this way and why it's based on the carbon isotope and all of that. But this is a fairly useful thing that is helpful to think about, what is the atomic mass likely to be? What is it roughly? When you see that hydrogen, it has one proton.

## Molecular mass and molecular weight

Clearly, that's what makes it hydrogen. It also has an atomic mass of one or roughly one, it says, "Well, it must not have a neutron. The electron also does contribute to mass but it's negligible compared to the proton. Most of this mass is essentially the proton. A little bit of the electron and also kind of the weighted average, when you think about the other isotopes of hydrogen that maybe might have a neutron that is involved with it.

For the sake of this, we could say, "Hey, hydrogen, it's a proton with an electron "kind of buzzing around it. Oxygen, same idea, we'll look at the periodic table. You could guess what its mass is close to. Oxygen, the most typical isotope of oxygen that you'll see on earth has eight protons. The eight protons are what makes it oxygen.

Any oxygen has to have eight protons anywhere that you are in the universe, if it doesn't have eight protons it's not oxygen.

Molecular mass and molecular weight - Chemistry - Khan Academy

It's gonna have eight protons and it's gonna have eight neutrons, that's the most typical isotope of oxygen. So you might guess that its atomic mass is 16 atomic mass units. You can see that your guess is pretty good. When we look at our atomic weight right over here. Which is the weighted average of the various isotopes of oxygen as found on earth. Our approximation to 16 is pretty good. Then based on these numbers you would say that this H2O has an atomic mass of approximately Well, two from the hydrogens, where did I get the two from?

Each of these two hydrogens have an atomic mass of one. If you have two times one, it's just gonna be two atomic mass units. Then 16 from the oxygen. Two plus 16, which is going to get us Let me do this in another color, since I've been using It's going to give us 18 atomic mass units. For example, water has a molar mass of The distinction between molar mass and molecular mass is important because relative molecular masses can be measured directly by mass spectrometryoften to a precision of a few parts per million.

This is accurate enough to directly determine the chemical formula of a molecule. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. December Learn how and when to remove this template message The term formula weight F. Precision and uncertainties[ edit ] The precision to which a molar mass is known depends on the precision of the atomic masses from which it was calculated. Most atomic masses are known to a precision of at least one part in ten-thousand, often much better  the atomic mass of lithium is a notable, and serious,  exception.

This is adequate for almost all normal uses in chemistry: The precision of atomic masses, and hence of molar masses, is limited by the knowledge of the isotopic distribution of the element. If a more accurate value of the molar mass is required, it is necessary to determine the isotopic distribution of the sample in question, which may be different from the standard distribution used to calculate the standard atomic mass.

The isotopic distributions of the different elements in a sample are not necessarily independent of one another: This complicates the calculation of the standard uncertainty in the molar mass. A useful convention for normal laboratory work is to quote molar masses to two decimal places for all calculations.