Posted by ~christina~ on Saturday, October 4, 2008 at 12:40am.
The only reason I asked about the balance is that most analytical balances in use in schools weigh to the nearest 0.1 mg although that may have changed since I retired. There are semi-micro balances that weigh to 5 places and micro-balances that weigh to 6 places but those aren't usually available to beginning students. And PLEASE don't call them scales. We use scales to weigh sacks of potatoes; we use analytical balances to measure mass in analytical chemistry.:-).
I am guessing that you are using the acidified dichromate solution to oxidize the alcohol and measuring the absorbance of the C=O group that is formed. As an added note about the balance, absorbance readings and spectrophotometric work usually is not accurate enough to justify four places for the oxidant (in this case the dichromate). Probably you could have used a triple beam balance just as well. Good luck on your experiment.
I think we were using the
balance with 5 digits.
Why is it usually not accurate enough to justify 5 places?
I'm just curious about that.
<I remember you saying that weighing the H2SO4 was not accurate because the density of sulfuric acid is not 1 or close to it. I looked it up and found it was 1.84g/cm^3>
I was trying to do the calculation of the sulfuric acid (10% wt/v) and I got lost as to how I was to measure out the volume of liquid.
I calculated the volume needed to be..
x/10 X100= 10%
x= 1g H2SO4
1g H2SO4 x (1cm^3)/1.84g = 0.54347ml
How would that not be accurate? I calculated I needed that much and I weighed out that much H2SO4. I don't know but I think I'm confusing myself, because I understood it before, and questioned this fact myself. I used the 1g of H2SO4 dissolved into 10ml of H2O in vol flask already though. Hopefully it doesn't affect the results too much.
Thanks Dr.Bob
- science(chem) Dr.Bob Question from before - DrBob222, Saturday, October 4, 2008 at 1:06am
You want an acidified solution and the H2SO4 need not be that accurate since it really never enters into the reaction. You CAN weigh 1 g H2SO4 but I would point out that since you didn't titrate the H2SO4 you don't really know its exact molarity (perhaps you did titrate it and it just didn't come up). The specific gravity listed on the bottle (see, I told you it was much heavier than water) is close but that's all. Just close. So again, you really don't need to weigh it to 5 places but it doesn't hurt anything to do that. You may be doing that just to give you practice. If you ARE using the acidified dichromate to oxidize alcohol to the aldehyde and measuring the C=O absorption, then you just need an excess of dichromate to make sure all of the alcohol is oxidized; i.e., the alcohol is the limiting reagent and you want to convert ALL of it to the end product. I hope I've addressed your questions.
- science(chem) Dr.Bob Question from before - ~christina~, Saturday, October 4, 2008 at 1:20am
Okay, I get it.
No it wasn't to 5 decimal places. I'm not looking at my notebook but it was 1.0---.
Thanks for your help
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