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### Topic: How convert mol % to wt%??  (Read 2415 times)

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#### ikkjkhhgs

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• Mole Snacks: +0/-0 ##### How convert mol % to wt%??
« on: June 15, 2018, 06:28:54 AM »
so the concentration of molecule A is 0.5% mol.

This is calculated by (number of molecules of A / total number of molecules) x 100

We are given that the molecular weight is 136 g/mol. I have no idea how to use this to convert this into wt%.

I have looked all over the interent and nothing has helped, the only solution from my research seems to be to convert mol% to mol (so it would be 0.005?) and then multiply that by the Mw, then divide by 10 (not sure why, something to do with units).

So it would be:

(0.005 x 136) / 10 = 0.068%

Pleease help, i’m really intrigued as to how converting to wt% works because it seems to be used a lot but I know nothing about it.

#### mjc123

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« Reply #1 on: June 15, 2018, 06:48:38 AM »
You can't work it out unless you know the molecular weight of the other molecules.

#### ikkjkhhgs

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« Reply #2 on: June 15, 2018, 07:07:11 AM »
You can't work it out unless you know the molecular weight of the other molecules.

Ohh okay. The other molecules are just water, so the Mw would be 18 g/mol I guess. How do you calculate from here?

#### mjc123

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« Reply #3 on: June 15, 2018, 07:25:45 AM »
Come on, this is simple. Suppose you have 1 mole of molecules in total. How many moles of A? How many moles of water? What is the mass of A? The mass of water?

#### ikkjkhhgs

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« Reply #4 on: June 15, 2018, 07:48:27 AM »
I’m not really sure what you mean. How can I calculate moles without a volume? I am ideally just looking for a formula or a guide that I can apply to my problem.

#### mjc123

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« Reply #5 on: June 15, 2018, 08:25:43 AM »
You don't need a volume. You have 1 mole of molecules. The mole fraction of A is 0.5%. So how many moles of A? You know the molecular weight, so what is the mass of A?
How many moles of water? What is the mass of water? What is the weight fraction of A?
Formulas are no use if you don't understand them.

#### Arkcon ##### Re: How convert mol % to wt%??
« Reply #6 on: June 15, 2018, 08:26:18 AM »
OK.  There may be formulas in use for every application, but not everyone knows them all.  It may be hard to find the formula you want, on the webpage you want, the the situation you have, each time.

However, understanding the underlying concepts, and using them, lets you determine the formula for every situation.  That's a smarter way to work.  That's what most people want to do.

I don't understand your jargon, and the way its calculated:

so the concentration of molecule A is 0.5% mol.

This is calculated by (number of molecules of A / total number of molecules) x 100

mol%?  What's that?  Mole fraction?  Molar?  Molar %?  And your calculation doesn't make sense to me either.

Quote
I have looked all over the interent and nothing has helped, the only solution from my research seems to be to convert mol% to mol (so it would be 0.005?) and then multiply that by the Mw, then divide by 10 (not sure why, something to do with units).

Here's something you can do.  Write the units, on paper, for each part of the conversion.  You can then cancel units, striking through, on paper.  So you can understand what that '10' means, or even if its applicable.

Come on, this is simple. Suppose you have 1 mole of molecules in total. How many moles of A? How many moles of water? What is the mass of A? The mass of water?

I’m not really sure what you mean. How can I calculate moles without a volume? I am ideally just looking for a formula or a guide that I can apply to my problem.

If you're in the mood to try this, pick a volume at random, like mjc123: picked a mass at random.  And see what you get.  Select the volume of 100 ml, that way, percentages are easy to find.  Also try 10 ml, and 1 mL.  Those are powers of 100, and easily convertible.

This is how we learn these concepts.  If you need to quickly make a solution to do some work this may seem tedious.  But you haven't given us enough context to help in that case.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

#### ikkjkhhgs

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« Reply #7 on: June 15, 2018, 08:44:13 AM »
You don't need a volume. You have 1 mole of molecules. The mole fraction of A is 0.5%. So how many moles of A? You know the molecular weight, so what is the mass of A?
How many moles of water? What is the mass of water? What is the weight fraction of A?
Formulas are no use if you don't understand them.

So would it be 0.5% = moles of A / 1 mole, = 0.5% (0.005 moles)

Then mass of A is 0.005 x 136 = 0.68 g ?

And that would mean 0.995 moles of water?

And thus mass of water being 0.995 x 18 = 17.91 g

So weight fraction would be 0.68 / 18.59 = 0.0366

And weight percent would thus be 3.66%?

#### ikkjkhhgs

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« Reply #8 on: June 15, 2018, 08:53:33 AM »
The calculation shows it is mole fraction, as it is the molecules of interest / the total molecules, which is then just multiplied by 100 to give mol %.

I apologise for any confusion, i’ve never done a calculation like this before and wasn’t sure what would be required. I didn’t want to give the whole question away as I wanted to figure it out myself (with help of course).

But the actual calculation is this:

Mol % A is 20 / (20 + 3980) = 0.005 x 100
= 0.5%

So there are 20 molecules of molecule A, and 3980 molecules of water. I am given that the Mw is 136 g/mol, and that these molecules are contained within a cube with 5 nm sides. This is all the information I am given to calculate wt%.

#### mjc123

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« Reply #9 on: June 15, 2018, 12:01:09 PM »
Your calculation looks OK to me.