Having Difficulty With Chemistry Conversion Problem. Please Help!!!!

[confused2021]

I am having difficulty with the following problem:

If you dissolve 360mg glucose (Formula Weight:  180.16g/mole) in 5mL of water:

 

 

 

1.What is the concentration of glucose in mM and µM?

I got 360 mg x 1mole/180.16 g = .002 moles

.002 moles x .005L = 1 x 10^-5 M x 1 x10^3 mM/1M = 1 x 10^-3 mM glucose

1 x 10^-3 mM x 1x 10^-3micromolar/1mM = 1 x 10^-5 micromolar glucose

 

2.If you take 5mL of this solution and dilute it to 500mL what is the concentration of the glucose in the resulting solution in µM? 

I used c1v1=c2v2

(1 x 10^-5 micromolar)(5mL) = (C2)(500 mL) = 1 x 10 ^-7 microMolar

 

3.How many nanomoles of glucose are present in 10mL of the diluted solution?

I  am having difficulty with number 3.  Can anyone tell me if numbers 1 and 2 are right and also how I can solve number 3.  Any input will be greatly appreciated!!!!

[Col Not Colin]

21 minutes ago, confused2021 said:

1.What is the concentration of glucose in mM and µM?

I got 360 mg x 1mole/180.16 g = .002 moles

.002 moles x .005L = 1 x 10^-5 M x 1 x10^3 mM/1M = 1 x 10^-3 mM glucose

1 x 10^-3 mM x 1x 10^-3micromolar/1mM = 1 x 10^-5 micromolar glucose

Hi.   I've only had time to scan through what you've written.  Thanks for putting your own answers in here,  we can see what you have done and what efforts you have made,  that's great.

In question 1, what you've done is reasonable but does seem to have a slight error.  This could be just an arithmetic or trivial error, in that case.....well, ok, we all have those.     BUT it could be a conceptual misunderstanding which we can do something about.  We can also suggest a method to check your conversions make sense and reduce the number of accidental errors.

   A Molar concentration is a concentration in  Moles  per  Litre.   (I think you did know that but if not, you need to know it).

The question has made things interesting by NOT making everything happen in 1 litre.    They said  ... dissolved in 5 mL.     (I think you did realise this).

When making conversions, it's a good idea to think about whether your final answer should be BIGGER or smaller after the conversion.  You have correctly worked out that there will be 0.002 Mols  of the glucose  in  5 mL.    Now is   1 Litre  more or less than  5 mL?     It is more than 5 mL, right?  Quite a lot more than 5 mL.

   So there should be more Moles of glucose in 1 litre then there was in 5 mL.     You have multiplied by  0.005  in this step, which will actually make the number of moles in 1 litre  smaller than the number in 5 mL.   That can't be right, can it?  

Can you adjust that conversion and then follow through with the conversion  of  Moles to milliMoles exactly as you did afterwards... and follow through for the micromolar units.... and the rest of the question paper.

 

Question 2 is ok   (BUT please use the corrected value from question 1).  There is another way you could do this, if you want a method to double-check your own answer.   Stick with what you have actually done, by the way, I'm sure that's what the teacher was looking for and that's what will be on the mark scheme.  It's sensible to make things as easy to follow for the person marking this as you can.  This alternative method is just a double-check for yourself.

    It just so happens, that you only dissolved the glucose in 5 mL to begin with.  When question 2 says... take 5 mL of this solution.... that is   ALL OF IT.   You already worked out the number of moles you had in those 5mL half-way through question 1   (that was 0.002 mols).   So the question is equivalent to saying  what concentration will you get when you have 0.02 Mols in a volume of 500 mL  (and make sure to convert to microMolar units at the end).

 

Question 3:      Break this into small steps.

Step 1:       Think...

What do you want?     The number of Moles.

What do you Know?     Information about Volume (you took 10 mL)       and........... that's not enough...  what else do we know?....

   it came from the solution we knew about in question 2.   What was the main thing we found out in question 2?    The concentration of that solution.

   Is that enough information?  We have   VOLUME,   CONCENTRATION     and  we want   NUMBER OF MOLES.       Are they related somehow?

HINT:   This is using the same idea you used (sadly, incorrectly) in question 1.     There is a formula connecting these three things,  do you know what that is?

 

Step 2:      You have a simple formula (mathematical formula),  use it to find the thing you want.    Remember to check the units you have used and the units they want in the final answer (like NANO-molar concentration) - do any conversions required.

Good Luck,  ask for more help if required.  

 

[Bufofrog]

1 hour ago, confused2021 said:

.002 moles x .005L =

This will give a result of mole-liters, that is not what you want.

1 hour ago, confused2021 said:

2.If you take 5mL of this solution and dilute it to 500mL what is the concentration of the glucose in the resulting solution in µM? 

You know the number of moles you have in the 500 mils, right?

 

1 hour ago, confused2021 said:

3.How many nanomoles of glucose are present in 10mL of the diluted solution?

You know how many miles are on the 500 mils, which means you know how many moles are on each mil, right?

Edited February 10, 2021 by Bufofrog

[Col Not Colin]

1 hour ago, Col Not Colin said:

So the question is equivalent to saying  what concentration will you get when you have 0.02 Mols in a volume of 500 mL

Hi again @confused2021.   I think I wrote 0.02 here when I meant 0.002.  Anyway, hopefully the spirit of the discussion is solid even if the typing isn't.  Also, isn't it great to know that other people can make mistakes?  Got to look on the bright side.

Also, @Bufofrog , I wish I had seen your reply first, would have saved me a lot of time typing.  (I'm also secretly hoping that you'll run out of time to correct your spelling of  Miles <--> Moles,  then we can all have a typo).

[Bufofrog]

47 minutes ago, Col Not Colin said:

Also, @Bufofrog , I wish I had seen your reply first, would have saved me a lot of time typing.  (I'm also secretly hoping that you'll run out of time to correct your spelling of  Miles <--> Moles,  then we can all have a typo).

I hate auto correct....