Homework Help: Chemistry Lab

Posted by Joe on Thursday, October 9, 2008 at 11:08pm.

Are you titrating? Qualitative testing? Just what? If the copper is in solution you can filter the ppt (or let the ppt settle and decant some of the supernatant liquid) and to the filtrate or the decanted liquid add some more NaOH to see if additional ppt will form. If I have guessed wrong about the application just clarify what you're doing and repost.

The lab pertains to the copper cycle. The reaction is copper nitrate (aq) + 2NaOH (aq) -> copper hydroxide (solid) + 2 sodium nitrate (aq)

If I had not added enough NaOH, I know I wouldn't be getting enough precipitate, but I'm not sure I would be able to tell just by looking at it. The copper nitrate itself is a light blue color and the end result should be dark blue with precipitate, but there is still some surrounding light blue solution

Chemistry Lab - DrBob222, Thursday, October 9, 2008 at 11:28pm
If there is a pale blue color to the liquid, I would be skeptical that all of the copper hydroxide had been pptd. You can do one of two things. (Of course, some of the ppt may be very finely divided, hasn't settled, and that could be providing the color.) One is to let the ppt settle for a period of time. Then add, dropwise and without stirring, NaOH to the liquid above the ppt. Observe closely if a ppt forms when the drops of NaOH make contact with the solution. The other way is to just add an excess of NaOH so you KNOW all of the hydroxide has pptd.

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