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Can someone check this for me - chemistry.

Can someone check this calculation for me, im usually alright at simple chemistry like this but i feel absolutely terrible, can't concentrate at all and just want to go back to bed! Its a calculation in a big writeup which if i get it wrong will screw up the rest of it so if anyone can check it for me i will be very grateful. As i said I have got an answer, im not looking for someone to do my work for me, just to check - I have this odd feeling that ive got it wrong.


0.5ml of 0.25mM Substance A is combined with 4.5ml of buffer solution to give a final concentration of substance A as 0.025mM

What is the amount of substance A in MICROmoles?

Thankyou
Reply 1
Avatar for cubanarmy
cubanarmy
1
vickyrkenya
Can someone check this calculation for me, im usually alright at simple chemistry like this but i feel absolutely terrible, can't concentrate at all and just want to go back to bed! Its a calculation in a big writeup which if i get it wrong will screw up the rest of it so if anyone can check it for me i will be very grateful. As i said I have got an answer, im not looking for someone to do my work for me, just to check - I have this odd feeling that ive got it wrong.


0.5ml of 0.25mM Substance A is combined with 4.5ml of buffer solution to give a final concentration of substance A as 0.025mM

What is the amount of substance A in MICROmoles?

Thankyou



n=MV/1000 M=0.25, V= 0.5+4.5=5
n=(0.5 x 5)/1000.

Don't have a calculator but you can do that bit.
Oh, the joys of amount of substance!
Reply 2
Avatar for Esquire
Esquire
1
Now, 5ml=5cm3 and 0.025mM=0.025milimoles/dm3
5cm3=5/1000 dm3 also 0.025mM=0.000025mol/dm3
moles=(concentration)x(volume)
=0.000025x0.005
=0.0000000125 moles
=0.0000125 milimoles
=0.0125 micromoles
Reply 3
Avatar for vickyrkenya
vickyrkenya
OP
2
thankyou both, I did get it right. So simple yet i just couldnt concentrate on that occasion. Will rep you both when i have some to give!
Reply 4
Avatar for vinny221
vinny221
11
I know this question is 2 weeks old but I actually got 0.125 micromoles lol...

There's 2 ways of approaching it:

Inital vol=0.5ml=0.0005dm3
0.25mM = 0.00025 moles dm^-3

moles =0.00025x0.0005=1.25x10^-7=0.125micromoles

OR

Total volume with buffer=5ml=0.005dm3
final conc =0.025mM=0.000025mol dm^-3

moles=0.005x0.000025=1.25x10^-7 moles=0.125micromoles

hope im not wrong on this one!
Reply 5
Avatar for oxymoron
oxymoron
7
vinny221
I know this question is 2 weeks old but I actually got 0.125 micromoles lol...

There's 2 ways of approaching it:

Inital vol=0.5ml=0.0005dm3
0.25mM = 0.00025 moles dm^-3

moles =0.00025x0.0005=1.25x10^-7=0.125micromoles

OR

Total volume with buffer=5ml=0.005dm3
final conc =0.025mM=0.000025mol dm^-3

moles=0.005x0.000025=1.25x10^-7 moles=0.125micromoles

hope im not wrong on this one!


Yes you are right!

The first poster got confused with units and the second poster just made a calculation error - correct working but has one too many 0's in the first answer (ie 3rd line from the bottom!) and that carries through.
Reply 6
Avatar for Esquire
Esquire
1
correct working but has one too many 0's in the first answer (ie 3rd line from the bottom!) and that carries through.

my zero button must be stuck or something... :biggrin:

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