Chemistry. Creating solutions.?

Question

Can someone please help me with this? Im so bad at chemistry and I've tried and tried.

You have a quantity of solid potassium dichromate from which you are to make 100ml of 4mM solution. Calculate how much potassium dichromate you must weigh out for this solution (molecular mass of potassium dichromate = 294.18g/mole)

That's the first part of the question and I worked it out to be 1.176727g, from rearranging n=m/M to m=nxM and then 0.004 moles x 294.18 g/mole is 1.17672grams.

I think I'm right on that part but then the second part is actually weighing it out, and then making the 4mM solution. I weighed out 1.19 grams so how much 100 mM sodium phosphate buffer do i need to add to create the 4mM solution?. (Im guessing you use C1V1=C2V2 but I dont see how)

thanks for any help guys!!

Answer 1

Be VERY careful

You want 4mM =0.004 mole/L
but also final volume 100 mL=0.1L
so you want n=C*V= 0.004*0.1= 4*10^-4 mole
also m=n*MW= (4*10^-4)*294.18 =0.11767 g

You said that the solution has to be 4 mM potassium dichromate.
Does it also have to be 4 mM phosphate buffer?
Make sure if you need to have the buffer and if the final concentration of the buffer should also be 4mM in the 100 mL volume. You said that the stock buffer solution is 100mM.
Then C1V1=C2V2 =>V1= (C2/C1)*V2= (4/100)*100= 4 mL
Note that here I didn't convert the units to mole/L and L because the convertion factors are simplified since I am expressing V1,V2 in the same units and C1,C2 in the same units.

So take a 100 mL volumetric flask or cylinder.
Put roughly 70 mL water. Put 4 mL of your 100 mM buffer, add 0.1177 g potassium dichromate, mix so that the solid dissolves, add water up to 100 mL (when you do that make sure that you have removed the stirrer- if you use one- so that the volume reading is accurate) and once you have added the water put back the stirrer and mix so that the solution is homogeneous.

If you meant that you will use the 100mM buffer as a solvent instead of water, then just put 70 mL of the buffer in the cylinder, add the solid, dissolve it, add buffer up to 100ml and mix.