In this video we are going to talk about how

to go from mass to mole fractions, and then from mole to mass fractions. So why would

you want to do something like that, well often you are given a stream and a molar flow rate,

but you are given the compounds and elements in mass fractions or mass percents, and you

need change them in order to figure out exactly how many moles you have of each element or

compound. So as I said we are going to start. We are going to go from mass to mole fractions,

and you are usually given this in fractions sometimes you are given it in mass percents.

It’s as you can see you do it the exactly same way. So let’s say we have a compound. That

36 percent by mass of carbon or 0.36, 16 percent hydrogen, 20 percent again by mass

of oxygen, and 28 percent or 0.28 mass fraction of nitrogen. So the first thing you do is assume a basis, now since these are mass fractions you’re going to want to assume

your basis in mass. You can choose whatever you want, but it often makes sense to choose

a number like 100, because what that means now is you have 36 grams of carbon, 16 grams

of hydrogen, 20 grams of oxygen, and 28 grams of nitrogen. The next thing you are going

to want to do is change your mass to moles, and in order to do that you use our old friend

the molecular weight or the molar mass. So if we have 36 gram of carbon, we divide it by

its molar atomic mass, 12 grams per mole, and that gives us 3 moles of carbon in our basis

of 100 grams. We do the same for the rest of our elements, and by the way here I am

doing it with elements, but you can do it with compounds as well. The processes is exactly

the same. So this is 16 moles, we have 20 grams of oxygen, we divide it by 16 grams per

mole and we get 1.25 moles, and finally our 28 grams of nitrogen is divided by 14 grams

per mole which gives us 2 moles. So these are the moles that are contained in a 100

grams of our compound. Step 3 is to add the moles together, and if we do that we realize that

we have 22.25 moles, and step 4 is to find the mole fractions, and if you remember the

definition of mole fraction. It is the mole, for example, of carbon divided by total moles.

So we are going to do that for each one. We have 3 moles of carbon. If we divide that by

22.25 we have 0.135 and that’s carbon. For hydrogen we have 16 moles we divide that

by 22.25 and we have 0.719 as a mole fraction. Oxygen, 1.25 moles divided by 22.25 equals

0.056 moles, and finally with nitrogen we have 2 moles we divide that by 22.25 moles

and that equals 0.090. So these are our mole fractions. One check you should always make

is to add up all these mole fractions and make sure it comes to one. Now we know how

to go from mass fractions to mole fraction. Let’s do the same except go from mole

to mass fraction, and we are going to start with the same percentages except remember these are

going to be mole fractions. So we have 36 percent carbon, 16 percent hydrogen, 20 percent

oxygen, and 28 percent nitrogen. So those are going to be our mole fractions. So now

again we start with assuming a basis, however, because these are mole fractions, again let’s

remember mole fractions. We should assume a basis in moles. So we are going to assume

a basis of 100 moles. The second step is to change the moles to mass. Now here with 100

moles as our basis, that means we have 36 moles of carbon and we multiply it by our

molar mass, which is 12 grams per mole, and we get 432 grams. We do that with hydrogen,

16 moles times 1 gram per mole which gives us 16 grams. Oxygen, 20 moles times 16 grams per

mole, which gives us 320 grams, and finally nitrogen, which is 28 moles we multiply that by

14 grams per mole and we get 392 grams. Step 3: add all the mass together and if you add

all the mass and is in this 100 moles of our compound we have 1160 grams, and our final

step is to find the mass fraction, which for example is the mass of carbon or hydrogen or

oxygen or nitrogen over the total mass. So let’s see what that looks like. We will start

with carbon there are 432 grams of carbon and we divide that by 1160 grams, and our

mass fraction is 0.372. Hydrogen, we have 16 grams we divide it by 1160 and our mass fraction

is 0.014. Oxygen, 320 grams divided by 1160 is 0.276, and finally with nitrogen we have

392 grams divided by 1160, 0.338. These are now our mass fractions and if we add them together

we end up with 1, and again similarly our mass and mole fractions are not necessarily

the same, and that has to do with the differing molar masses.

omg this helped me sooo much!

Thank you so much! This is extremely helpful

Very good

the elements should be diatomic right?

do you do mass balances? with boundaries and degrees of freedom please? Assignment coming up,.hehe Thank you!

how do you do a mass balance using volume as basis?

Thank you so much, you really helped me to do my homework. i appreciated it… THANKS once more

Thank you very much. It is always nice to find a straightforward and short explanation when I need to fresh knowledge.

Thanks !!! you helped me a lot than my teacher.

thank you so much.I learned something really

How to solve a gas sample contain argon and oxygen gases and has a total pressure of 2.35atm. IF the partial pressure of argon is 865 torr, what is the mole fraction for each of the gases in the mixture? Help

i dont get the difference between mass fraction and mass precent lol , are they the same

need assistance with converting mass % composition to mole fraction and set up tables of composition, break temperature, arrest(halt) temperature for diphenylamine at 100% adding 10g naphthalene, 83.3% adding diphenylamine, 66,7% adding diphenylamine please assist.

im confused about when to use mass or mole fractions

Thank you so much for your lesson of mass to mole fraction.

Thank you

@LearnChemE…. can we consider mole fraction as volume fraction?