soulnoob - 20-9-2008 at 03:07
hi veeryone,
i just did this experiment last week but i have couples of question regarding about it,can anyone explain to me?
by dissolve 5g of CuSO4.5H2O in 30 cm3 of water,i get a aquaoues solution of CuSO4.5H2O.then added 10 cm3 of NH3 into the solution,
CuSO4.5H20 + NH3 -----> Cu[(NH3)4]SO4.H20 + H20(this solution has a deep-blue color)(i calculated CuSO4.5H2O to be the limitting reagent)
Then i added 40cm3 of ethanol in the solution,the solution precipitate(why???)
Then i filter the solution and get a blue-purple salt(i assume that it is the salt Cu[(NH3)4]SO4 but not sure)
Here is some number:mass of CuSO4.5H2O before:5.0675,mass of the precipitate after filtered:4.6172
Can anyone explain??
Thx
Pomzazed - 20-9-2008 at 03:18
1)Then i added 40cm3 of ethanol in the solution,the solution precipitate(why???)
Ethanol has lower polarity comparing to water. thus adding it lowering the ability of ionic species to be soluble in it. thus they precipitate out.
2)Then i filter the solution and get a blue-purple salt(i assume that it is the salt Cu[(NH3)4]SO4 but not sure)
Should be, the color is darker than just CuSO4.5H2O
3)Here is some number:mass of CuSO4.5H2O before:5.0675,mass of the precipitate after filtered:4.6172
- Dont forget to calculate the mass change from hydrate (5H2O -> monohydrate).
- You wont get all Complex precipitate out of solution, some of it still swirl floating happily in the solution
- Expect mass lost in the workup steps (stick to filter paper, equipments, etc)
soulnoob - 20-9-2008 at 04:47
thank you for your help and one more quesion,CuSO4.5H20 + NH3 -----> Cu[(NH3)4]SO4.H20 + H20 which one is the limiting reagent?i calculated it to
be the copper sulphate but i read on the net it does say that you need excess of NH3?does this it NH3 is the Limitting reagent?
Klute - 20-9-2008 at 06:51
The complex formation is an equilibrium. I fyou add only 0.1excess for example, you might no have all the Cu2+ complexed. So you add something like
10x exces NH3 to be sure you get all the copper as the tetrammine complex. This why you did have the theoritical amoutn of complex. And there will
always be a little left dissolved, aq. EtOH solution do dissolve ionic species to a certain extent depending on the % of water, especially such
coordinated species.
Add at least 5x excess NH3, and add a little more EtOH, and place it in the fridge to get the most out of it!