Okay so heres the question

To prepare 25 ml of a 50 mM solution of the ester methyl ethanoate, what volume of methyl ethanoate
(in µl) should be dissolved in water

methyl ethanoate density = 0.93
g/ml

so far what i've done is:

50 mM = 0.05 M

1 ml of
methyl ethanoate = 0.93g
25 ml of methyl ethanoate = 23.25g
RMM of methyl
ethanoate: 74

Moles = 23.25/74 = 0.314

Volumme (L/dm^3) =
0.314/0.05 = 2.68 L

which can't be right
My chemistry isnt great so if
anyone has any advice?

 

The question isn't asking you to make a 50 mM solution from 25 mL of your ester, it's asking you to make 25 mL of a 50 mM solution of your ester; in other words, the 25 mL should be 50 mM. You need to work out how many moles of methyl acetate would be in 25 mL of that solution and go from there.

I'm not entirely sure on what you're suggesting me to do here.

Is any part of what I worked out correct?

No.

Sorry, I take that back. You correctly converted mM to M. Your working out is right with everything else, but you're not answering the question it's asking you, so all your numbers and steps are wrong.

 

I'll post a more detailed explaination if I have time later. For now, think about this. Why would you assume that the question is telling you that you have 25 mL of methyl acetate when it's asking you what volume of methyl acetate you need?

Hmmm okay thank you I'll post back later if I manage to work it out or not.

 

Thanks for your help so far though

I'm still struggling to get the answer here so any further hints on where I should stard would be much appreciated.

 

Thanks

Edited February 10, 201312 yr by Copyright

OK. The aim of the task that the question is asking you is to make up a certain volume of a 50 mM solution of some compound. So, if you wanted to make 1L of that solution, you would need 50 mmol or 0.05 moles of whatever that compound is in 1L of solvent. Let's say we don't need 1L. Let's say that we only need 0.5 L, or 500 mL. To do that, we only need 0.5 x 0.05 moles, or 0.025 moles of your compound made up to 0.5 L with your solvent. Similarly, if you wanted 2L, you would use 2 x 0.05 moles, or 0.1 moles of your compound.

 

In your question, you only need 25 mL of solution, or 0.025 L. So how many moles of your compound do you need to make up that much solution?

Thank you I have the answer!

Edited February 10, 201312 yr by Copyright