[factot_20]

I have a 5ml of 50mM of ascorbic acid. I have to prepare the solution of different concentration. First, i took 0.05ml of ascorbic acid and add 0.95ml of water in it to dilute it. what is the concentration of ascorbic acid in this dilute solution. can anybody please explain. Thanks a lot in advance!

[Corribus]

(You should be honored - My 1000th post!)

There are shortcuts you can take to determine this, but let's do it the full-proof way.  Molar concentration is the number of moles of your solute divided by the total volume of solvent.

Here you have 50 mM ascorbic acid stock solution, and you're taking 0.050 mL of this and adding it to 0.95 mL water.

The denominator I think you should be able to figure out. So, now you have to figure out how many moles of your solute (ascorbic acid) are in 0.05 mL of a 50 mM solution = your numerator.

[factot_20]

(You should be honored - My 1000th post!)

There are shortcuts you can take to determine this, but let's do it the full-proof way.  Molar concentration is the number of moles of your solute divided by the total volume of solvent.

Here you have 50 mM ascorbic acid stock solution, and you're taking 0.050 mL of this and adding it to 0.95 mL water.

The denominator I think you should be able to figure out. So, now you have to figure out how many moles of your solute (ascorbic acid) are in 0.05 mL of a 50 mM solution = your numerator.

According to you,

mM of ascorbic acid = number of moles of ascorbic acid/total volume of solvent (water)
                             
number of moles of ascorbic acid = 0.05 x 50 = 2.5 mmol
therefore,  concentration of ascorbic acid = 2.5/0.98 = 2.55 mM

is this right?

[Corribus]

Where are you getting 0.98 from?

[factot_20]

Where are you getting 0.98 from?

Oh sorry it was 0.950..is that correct the way I calculated?
And I also want to ask about the percentage uncertainty. I used p200 (20-200μL) mocropipette for measuring ascorbic acid and p1000 (100-1000μL) for measuring water. What is the error involved? I know how to calculate the percentage uncertainty after that but I can't determine the error in micropipette
If you could help please. Really appreciated!!

[Corribus]

Look again at what your total volume is. You're adding 50 microliters of your stock to 950 microliters of water. 

Re: error. The pipette itself probably has an inherent tolerance that you can look up in the documentation. At worst, call the company. In my experience, user error is a far greater source of inaccuracy than the pipette itself - a surprisingly large amount of people don't know how to use a pipette correctly.  That said, pipettes do need periodic calibration. If it's been more than a year (or less, if you have a QC SOP that calls for it) since your pipette has last been calibrated, it may be time to do it again. You can get a rough idea of accuracy gravimetrically, if you have a good calibrated balance.

[factot_20]

Look again at what your total volume is. You're adding 50 microliters of your stock to 950 microliters of water. 

Re: error. The pipette itself probably has an inherent tolerance that you can look up in the documentation. At worst, call the company. In my experience, user error is a far greater source of inaccuracy than the pipette itself - a surprisingly large amount of people don't know how to use a pipette correctly.  That said, pipettes do need periodic calibration. If it's been more than a year (or less, if you have a QC SOP that calls for it) since your pipette has last been calibrated, it may be time to do it again. You can get a rough idea of accuracy gravimetrically, if you have a good calibrated balance.

For the first bit, molarity is moles of solute/Litre of solution.
Molarity = 2.5mmol/1mL = 2.5 moles/litre = 0.025 mM
Is that correct now?

[Corribus]

Seems like it.

[DrCMS]

For the first bit, molarity is moles of solute/Litre of solution.
Molarity = 2.5mmol/1mL = 2.5 moles/litre = 0.025 mM

"Molarity = 2.5mmol/1mL = 2.5 moles/litre" yes that is correct but it is not the concentration of the solution you have prepared and 2.5 moles/litre ≠ 0.025 mM

Try your maths again. 

How many moles of ascorbic acid are in the 0.05ml you took from the 50mM solution?
How much volume is that no of moles now dissolved in?
So what is the final concentration?

[Corribus]

Right, I missed that. Guess that's what I get for reading quickly.

[factot_20]

For the first bit, molarity is moles of solute/Litre of solution.
Molarity = 2.5mmol/1mL = 2.5 moles/litre = 0.025 mM

"Molarity = 2.5mmol/1mL = 2.5 moles/litre" yes that is correct but it is not the concentration of the solution you have prepared and 2.5 moles/litre ≠ 0.025 mM

Try your maths again. 

How many moles of ascorbic acid are in the 0.05ml you took from the 50mM solution?
How much volume is that no of moles now dissolved in?
So what is the final concentration?

it would be 2.5mmol/0.950mL = 2.63 mol/L
is that correct?
i am so bad in chemistry!! can't figure it out.

[DrCMS]

No.

What does 50mM mean, 50 milimoles per what?

So how many moles are in 0.05ml or 0.00005L?

If you add 0.05ml (of a very dilute aqueous solution) to 0.95ml of water what is the final volume?

[factot_20]

No.

What does 50mM mean, 50 milimoles per what?

So how many moles are in 0.05ml or 0.00005L?

If you add 0.05ml (of a very dilute aqueous solution) to 0.95ml of water what is the final volume?

ok 50mM is 50 milimoles per litre.
1 litre contains 50 moles
0.05 ml (0.00005) contains moles = (50/1000) x 0.00005 = 0.0000025 moles = 0.0025 m moles
total volume = 1.00 ml
concentration = 0.0025/1 = 0.0025 moles/litre
how about this?

[DrCMS]

ok 50mM is 50 milimoles per litre.

Yes

1 litre contains 50 moles

No 1L contains 50milimoles or 0.05 moles

0.05 ml (0.00005) contains moles = (50/1000) x 0.00005 = 0.0000025 moles = 0.0025 m moles

but this bit is right.

total volume = 1.00 ml
concentration = 0.0025/1 = 0.0025 moles/litre

the final answer is right

[factot_20]

ok 50mM is 50 milimoles per litre.

Yes

1 litre contains 50 moles

No 1L contains 50milimoles or 0.05 moles

0.05 ml (0.00005) contains moles = (50/1000) x 0.00005 = 0.0000025 moles = 0.0025 m moles

but this bit is right.

total volume = 1.00 ml
concentration = 0.0025/1 = 0.0025 moles/litre

the final answer is right

Thank you,
can't i write mol/litre as M??

this is from wikipedia
''The SI units for molar concentration are mol/m3. However, most chemical writing uses mol/dm3, or mol dm-3, which is the same as mol/L. These units are often written with a capital letter M (pronounced "molar"), sometimes preceded by an SI prefix, for example, millimoles per litre (mmol/L) or millimolar (mM), micromoles/litre (µmol/L) or micromolar (µM), or nanomoles/L (nmol/L) or nanomolar (nM).''

i am so confused because i want to express my results as M