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
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.
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
In analytical chemistry, the number of decimal places used in measurements is typically dependent on the accuracy required for the analysis being conducted. While it is possible to use a balance with five decimal places, it may not always be necessary or justified.
In the case of absorbance readings and spectrophotometric work, the accuracy needed is usually determined by the instrument being used and the inherent variability in the measurement technique. Generally, spectrophotometers have limitations in terms of the accuracy and precision of their measurements, which may not be sufficient to justify the use of five decimal places on the balance.
Using a triple beam balance, which typically has three decimal places, could be equally satisfactory for most analyses. It provides sufficient precision while not introducing unnecessary uncertainty due to the limitations of the spectrophotometer.
Regarding your calculation for measuring the volume of sulfuric acid, your calculation is correct in terms of determining the volume needed based on the weight of the acid and its density. However, it's important to note that measuring small volumes, especially in the range of milliliters, can introduce errors due to inaccurate pipetting or handling of the liquid. These errors can affect the accuracy of the overall analysis.
In your case, using a 1g H2SO4 dissolved into 10ml of H2O in a volumetric flask should give you a reasonably accurate solution for your experiment. While the accuracy may not be perfect due to potential measurement errors, it is generally acceptable for many analytical techniques.
It's always advisable to consider the required accuracy for your specific experiment and use appropriate techniques and equipment to minimize any potential measurement errors.