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#### eoerex

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« on: September 28, 2014, 06:12:53 AM »
Hi everyone, I'm having trouble with this question:

"Calculate the number of moles required to make 250ml of 100mM KMnO4
Give your answer in moles to two significant figures omitting the units"

I didn't do A-Level chemistry only Biology, and I've watched a few videos and read a few articles on how to get the atomic mass etc and it's embarrassing as to why I don't quiet know what this question is asking of me. I know the molecular mass is 158.

I'm sorry to ask such a basic question. one part that i'm confused upon is the 100mM is that 0.1?

Any help would be very much appreciated as I also need to answer the sister question which is
"Now that you know how many moles of KMnO4 are required and you know the molecular mass, you should your previous answers to calculate what mass of KMnO4 you require to make 250ml of 100 mM"

thank you for your time and again, any help would be so very much appreciated

Kindest regards

#### Hunter2

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« Reply #1 on: September 28, 2014, 07:00:04 AM »

1. What is molar mass of KMnO4
2. Do you know what does mM means?

If you can answer this. Then you can calculate the sister question.

#### eoerex

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« Reply #2 on: September 28, 2014, 07:29:43 AM »

1. What is molar mass of KMnO4
2. Do you know what does mM means?

If you can answer this. Then you can calculate the sister question.

I think by what I'm reading, is that the molar mass is 158.03395g being - K:39.0983 Mn:54.93805 O:15.9994

But What does mM mean then? this is where I'm confused

Thank you for your reply though, sorry I'm a mature student and trying to get my mind around it having not done it before.

#### Hunter2

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« Reply #3 on: September 28, 2014, 07:32:59 AM »

for second one. M is equal molarity. Do you know what that menas and what unit it has?
and the samll m in mathematics. What does mm, ml, mg, etc. means.

#### eoerex

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« Reply #4 on: September 28, 2014, 07:55:01 AM »

for second one. M is equal molarity. Do you know what that menas and what unit it has?
and the samll m in mathematics. What does mm, ml, mg, etc. means.

Ah, ok thank you, well I'm not sure what M exactly means and certainly not when it is mM, i understand that measurements of mm, ml, mg, so m would be meters? Or am I being horrendously dense minded?

Thank you

#### Hunter2

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« Reply #5 on: September 28, 2014, 08:07:43 AM »
M = Molarity. Its the moles in 1 liter solution.  m = milli = 0.001

1 mM = 0.001 mol/l

So how much would be 100 mM?

#### eoerex

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« Reply #6 on: September 28, 2014, 08:16:34 AM »
M = Molarity. Its the moles in 1 liter solution.  m = milli = 0.001

1 mM = 0.001 mol/l

So how much would be 100 mM?

Oh ok, thank you Hunter2. So would I be right in saying 100mM is 0.1 mol/l?

#### Hunter2

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« Reply #7 on: September 28, 2014, 08:18:40 AM »
Yes correct and now you can answer the original question. How many moles you need for 250 ml and also answer the sister question.

#### eoerex

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« Reply #8 on: September 28, 2014, 08:39:41 AM »
Yes correct and now you can answer the original question. How many moles you need for 250 ml and also answer the sister question.

Thank you Hunter2, I really appreciate your *delete me*

#### Hunter2

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« Reply #9 on: September 28, 2014, 08:41:10 AM »
What is the result.

#### eoerex

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« Reply #10 on: September 28, 2014, 09:13:32 AM »
What is the result.

Ok right, so I think without hopefully being too embarrassed about getting it wrong, but, theres 0.25 mol/l required for the 1st question and then is it the mass 158.03395 x the mol/l = 39.5084875g?

#### Hunter2

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« Reply #11 on: September 28, 2014, 09:17:09 AM »
No....calculate again.

How it can be 0,25 mol/l if you have only 0,1 mol/l. Check also units its alo wrong.

#### eoerex

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« Reply #12 on: September 28, 2014, 11:38:54 AM »
No....calculate again.

How it can be 0,25 mol/l if you have only 0,1 mol/l. Check also units its alo wrong.

Sorry, grrr, I'm getting confused again, so is it 0.025 mols? I'm clearly not understanding the question perhaps? Sorry for being dense

#### Hunter2

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