Question:

How do I improve my lab technique?

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I've never been very good in chemistry lab but as I take more advanced chem classes they're becoming less forgiving with how they grade lab reports. Right now I'm taking quantitative analysis and they're really strict because they will give you and F on a lab if your analysis is off by more than a few percent. Today I got my second F. We did a chloride analysis and I had everyone had between 50 and 60% chloride in their unknown. I got around 54% and my instructor told me I was way off and failed me. I've tried doing everything slowly and paying very careful attention but it's not working. Does anyone have any tips?

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3 ANSWERS


  1. YES. Talk to the lab boy who gave out the unknowns. Get a new one. I did 32 determinations of CL in Hudson Valley Community College with the last 4 having the department chairman looking on before he stopped 'F' ing me and admitted that maybe the unknown was screwed up.


  2. First, you have to know what you are doing.  Read the lab instructions at least twice before you walk into the lab.  If you have questions write them down and get answers from your professor before the lab starts.  Organize your work space before you start working, put all of the equipment and supplies you will need where you can access them easily.  Second, you have to work carefully.  Pay attention to your lab instructions and follow them carefully.  Work at a pace that is comfortable for you, this isn't a race and working too slowly won't make your work any better.  Take notes if you need to keep tract of where you are in the procedure.  Keep your bench clean, wipe up spills and keep your supplies and equipment in order.  

    Third, do a complete and thorough write-up.  Check all of your calculations twice, nothing will make you look dumber than an error in arithmetic.  If your lab has a standard form for the report use it and be certain that it is complete.  If there is not standard form make sure your report is concise and free from errors in grammar and spelling.  If you have to give a hand-written report make sure that it is legible.

  3. Some procedures are a little trickier to master than others.  I've found it helpful to make sure I understand exactly what's going on at every point, so that I know how things *should* look.  Work slowly (but efficiently - running out of time is just as bad), to avoid making mistakes out of haste.

    You didn't mention anything about the type of analysis; I'll assume it was a titration.  (If your class is still doing it grav., tell your instructor to get his head out of the 50s.)  The usual errors come from having the titrant concentration be off by a bit, or missing the endpoint.  (Or perhaps you didn't dissolve the solid sample completely?  It'd be more helpful if he told you whether you were too high or too low...)

    Run blanks and standards.  Watch the endpoint closely as you standardize the titrant - make sure you know exactly what the change looks like.  Make sure you're going slowly enough; sometimes the precipitation reaction is a little sluggish, so one drop at a time is plenty!  Keep separate little beakers of "before", "after" and "at" the endpoint handy as you work - don't discard the earlier solutions until the last one's done.  (That way you're always titrating to the same endpoint.  It may be the wrong endpoint, but at least you'll be more consistent and hopefully your RSD will drop.  Or was it already pretty low?)

    Get some local help.  You should have TAs, or else the instructor needs to be there anyway... have one of them watch your procedure carefully and see if they've got any ideas for what you're missing.  (They're paid to teach you, not just babysit so you don't blow up.)  Compare notes and techniques with the other students - I assume you all have different unknowns, but maybe they're doing something a bit different?  Find someone who always gets it right and watch him/her for a bit.

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