Have I done this right? - (Jul/21/2006 )

Hi there

Can someone please tell me if I have the correct working/answer?

1)..If you have 0.2g of haemoglobin (Mr=67,000) and you dissolve it in 500ml of buffer, calculate the conc. in um of this haemoglobin solution.

So I went...n=m/M=0.2/67,000=2.99x10^-06mol
then using c=nv=(2.99x10^-06)x(500/1000)=1.49x10^-06M=1.49um

2)..How many mg of haemoglobin would you need to dissolve in 800ml of buffer so that you have a solution that has a conc. of 5uM?

So I went...n=cv=(5x10^-06M)x(800/1000)=4x10^-06mol
then m=nM=(4x10^-06mol)x(67,000)=0.268g=26.8mg

3)..A solution of cytochrome c (Mr=12,400) has a concentration of 10umol/L. Calculate the amount (in mg) in 5L.

I am unsure about this one...just in general when giving a concentration value I'm kinda confused cos seeing as c=n/v then shldn't c be given as eg: mol/L so then how come in my notes it says mM is a concentration??? and see how for question 1) it asks for the conc. in um but in question 3) it gives us a concentration of 10umol/L..........can someone please explain my confusion??

Thank in advance
biology_06er

hi

Hence, 10µM with 12 400 g/mol is 12 400 x 10^-6 for 1liter

So for 5L : m = 5 x 10^-6 x 12400 = 0,62 in grams. So 620mg.

1)

you have 0.2g in 500 mL

your concentration is 0.4g/L
now how many moles are in 0.4g

mole = mol/g*g

mol/g is (g/mol)^-1. id est 1/67000

then in 1 liter you have 1/67000 *0.4= 6*10^-6 moles.

the concentration is 6µM.

2)

you want to know how many mg add to 800 mL to get a 5 µM solution

5 µM = 5 µmol/L

how many mg in 5 µmoles?

you have 67000 g per mole, then in 5 µmoles you have 5*67000 µg = 335000µg
This is in 1L, but in 0.8 L you need to put less. id est 335000*0.8 = 268000µg

you have to add 268 mg in 800 mL to get a 5 µM solution

your mistake is that you use formulas but it seems they are not helping you, become it seems ...
ah sorry, I don't know how to say it in English.
and then you do mistakes, like *500/1000 instead of *1000/500.
it's not concrete enough.

I don't know if it would be better for you, but why don't you try to do it the way I did? I don't say it's better, it's just an other way to do it.

and if your professor prefers that you use formulas, then use formula, and check by an other method that your formula is alright.


and yes, 1M is 1 mol/L. just two different ways to say the same thing