 This topic has 16 replies, 4 voices, and was last updated 9 years, 4 months ago by JackBean.

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March 23, 2011 at 12:51 pm #14712kitkat11Participant
I’m confused with working out the final concentrations of these.
I added these to 6 separate flasks each containing 40 ml of ecoli in a glycerol medium:
a. 2.0 ml (10 mM) IPTG
b. 0.6 ml (1%) lactose
c. 0.6 ml (1%) lactose and 0.5 ml (8%) glucose
d. 0.6 ml (1%) lactose and 1.0 ml of 5FU
e. 0.6 ml (1%) lactose and 1.0 ml of Chloramphenicol
f. control (no adds)ive confused myself beyond doubt with the final concentrations of each of these additions in writing up my legend. .. please help!
ive already thought of doing these methods but they dont feel like they are right :
E.g If I am adding 0.6 mml of 1% lactose to a 40 ml culture of E.coli bacteria
Method 1: 1% of 0.6 ml is 0.06/40 = 0.0015 mM which would be 1.5 uM (i hate concentrations if you noticed? )
Method 2: c=n/v …. molecular weight of lactose is 342/(40 ml + 0.6 ml) = 8000 something which i would thing is wrong? unless it is 8423 uM and then it becomes 8.4 mM ?
Method 3: im running out of ides –> 0.6/0.01 = 60 mM?
PLEASE HELP ME WITH A, B, C, D, AND E!

March 23, 2011 at 1:10 pm #104128JackBeanParticipant
1) you have 1% solution, so you canNOT get 0.0015 mM just from V1/V2!!!
2) use simply c1V1 = c2V2, you just need to keep the units of concentration (c1 and c2) the same, i.e. either % or M, but do not mix it! 
March 23, 2011 at 1:57 pm #104129kitkat11Participant
yes i noticed that mistake but i was so confused let people see where i was going wrong … umm can u explain that formula u gave though in a bit more detail (or e.g.) because i’ve never seen it? or heard of it? thanks!

March 23, 2011 at 5:24 pm #104131canalonParticipant
c1v1=c2v2
When calculating a concentration if you multiply the volume of original solution used (v1) by its concentration (c1) it is equal to the final concentartion (c2) time the final volume (v2)
i.e:
if you mix 1ml(v1) of a 1M solution (c1) in 100ml(v2) the final concentartion is c2=(c1xv1)/v2 or c2=0.01MBeware your units if C1 is in %, c2 will be in % and if c1 is in Moles/L so will be c2
Knowing the Molecular weight of your compounds is necessary to convert between Moles and %

March 23, 2011 at 9:36 pm #104135kitkat11Participant
ok so i think i get that formula … but how does moles then relate to percentage. so like for lactose it has a molecular weight of 132 (roughly) and the only formulas im thinking of include n=m/M and c=n/v …. but i don’t know how that relates to percentages?

March 23, 2011 at 10:05 pm #104137canalonParticipant
percentages are usually given v/v(volume to volume ratio) or w/v(or weight to volume ratio).
The first is used for liquids (water, ethanol, detergent like triton…) and is the ratio of the substance to the solvent. i.e. a 13% v/v ethanol solution (aka wine) contains 13ml of ethanol for a final volume of 100ml
The later is used for solids (salts, sugar,…) and represent the weight of solute(in grams) in a 100ml final solution. i.e. a 10% solution of arabinose contains 10g of arabinose in a 100ml volume. This is the kind of thing you are dealing with.
Now since you can calculate the number of moles in your solution since you know the amount of your solute and its molecular weight, you will have no problem calculating your concentrations (remember 100ml =/= 1L)

March 23, 2011 at 11:51 pm #104139kitkat11Participant
ok so i think i understand … im just want to check.
so if i do the 0.6 ml of 1% lactose and put that in 40 ml of ecoli culture…. then:
i take 0.001 / 132 = 7.57 x 10^6
and then i times that by 0.006 = 4.5 x 10^8
and divide that by 0.0406 = 1.1 x 10^6 which would be 1.1 uM?sorry … im just having trouble now applying what you’ve said.
thanks so much though!

March 24, 2011 at 12:19 am #104143canalonParticipant
c1 x v1=c2 x v2
with
c1=1%
v1=0.6mlc2= you will calculate it
v2=40.6 mlYou have a concentration in % then you just convert that percentage in molarity by using the molecular weight of lactose (342.3g/mol)
1% = 1g/100ml= 10 g/L =10g/(MW of lactose in g/mol) mol/Land your result is wrong by many order of magnitude.
Your result is 
March 24, 2011 at 8:02 am #104150JackBeanParticipant
I will explain the formula a little bit more.
as you wrote, you have n = c.V
so, if you dilute your solution, you add only pure solvent and thus the amount of your solute is not changing, thus if n1 is the amount before dilution and n2 after dilution, it is n1 = n2 and thus you can write
c1 . V1 = c2 . V2this is for the moles and mol/dm3, but the same applies for the %, because if it is true, that n1 = n2, than must be true m1 = m2 (if we’re talking about the same substance and thus the same Mr;) and since
m = c . V
because in this case c is in [%] (w/v), that is, as canalon wrote, how many grams you have per 100 ml of solution
so you get again
c1 . V1 = c2 .V2
(but remember, that both c are in %!) 
April 1, 2011 at 12:50 am #104246kedw4123Participant
so, can someone please give an example using B
Thanks! ๐

April 1, 2011 at 1:04 am #104247kedw4123Participant
so is this right for B:
c1 x v1 = c2 x v2
c1=1%
v1=0.6
c2=what we find
v2=40.6MW of lactose =342.3g/mol
1%= 1g/100ml
= 10g/L
=10g/342.3g/mol
=0.029 mol/Lc2=c1 x v1/v2
= 0.029 M x 0.6 ml / 40.6 ml
= 0.0043 mol/L
=0.43 ยตMIs that correct? Should the concentration be expressed like that?
Also, I’m having trouble on D
0.6 ml (1%) lactose and 1.0 ml 5fluorouracil (FU)
Please help!!!

April 1, 2011 at 1:25 am #104248kedw4123Participant
btw…following c2=c1 x v1/v2, my calculations are out,
I meant 0.029 M x 0.6 ml /40.6 ml
= 0.00043 mol/L
=0.43 mM 
April 1, 2011 at 7:26 am #104252JackBeanParticipant
why are you calculating with the molecular weigth? Just use the percentage ๐
c2 [%] = (1%*0.6ml)/40.6ml
what’s that hard about that?

April 1, 2011 at 8:43 am #104257kedw4123Participant
If I do that, my answer is 0.015….but what are my units :S

April 1, 2011 at 9:36 am #104259JackBeanParticipantquote JackBean:c2 [%] = (1%*0.6ml)/40.6ml

April 1, 2011 at 1:22 pm #104265kedw4123Participant
:/

April 1, 2011 at 7:30 pm #104271JackBeanParticipant
let’s say, you will add 2 ml of 5% solution to 98 ml, OK?
So it will be
2 ml * 5% = (98+2) ml * c2 [%]
c2 = 0.1%


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