Chemistry - Figuring uncertainty

Our group had to create a procedure that efficiently separated mixtures of sand and salt. This we did, and all of the samples separated perfectly, except one, which was a gram off. On this one, my filter had broken as I was running water over the sample, and I ended up with one gm more of sand and one gm less of salt. I reconstituted the mixture, and came out with the same result, even though all of the other mixtures came out perfectly (we separated 16 different mixtures).

Now, we are not sure if we have calculated the uncertainty correctly, and my teacher won't respond to our questions on figuring it (the class is online). This is what I figured - is it correct? Am I missing anything? should I list human error under uncertainty in our spreadsheet? Thanks!

"One source of uncertainty was that our scales only weighed to the nearest gram, so the sand or salt’s original weight might actually have been from 29.5g - 30.4g, but the scale would still read 30g. The same would be true for reading the weight of the pan. To reduce this uncertainty, we sampled 16 mixtures – four samples from each of four different people, with each person having a specific percentage of sand and salt mixture, different from the other student’s samples, that they sent out to three other students, keeping a fourth sample for themselves. This uncertainty, however, would account for the one gram overweight of the sand in Mary’s test sample from John. The loss of one gram of salt might also have been from this weight uncertainty. In addition, when testing this sample, the filter broke, allowing about half of the wet sand and salt mixture to fall into the pan, necessitating a refiltering of the mixture. In falling, a small amount of the salt/water solution may have splashed out of the pan. This source of uncertainty would stem from human error. If we were to do this experiment again and would attempt to eliminate the possibility of human error, we might want to send two samples of each mixture to each person so that there would be a backup sample in case one sample was spilled or otherwise damaged."

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  1. I would use "experimental" error instead of "human".

    The first part is wordy, and somewhat difficult to follow.

    You apparently were dealing with a salt and sand mixture, trying to separate the salt from the sand.

    If you "lost" one gram of salt, and "gained" one gram of sand, is it possible a gram of salt was left in the sand? I am not certain how you washed and dried the samples. I have seen students just remove the wet sand from the filtrate, then dry the sand (leaving some salt in the sand).

  2. We used a coffee filter in a funnel to separate the sand and salt, and rinsed the spoon and the funnel and the outside of the filter before we evaporated the water. I suppose there is a possibility of leaving a gm of salt in the sand mixture, but I reconstituted the mixture and filtered and dried it again with the same results. We also separated 15 other mixtures, which all came out perfectly. I know it is a bit wordy, but my professor is very uncommunicative about what she wants and randomly docks points for things she never told us should be on our assignments in the first place. She just docked me a point on an assignment for answering a direct question from the text with the text's answer - she said my answer was wrong, so I guess the text is wrong too. I'm 51 and have a bit of OCD, so I'm pretty careful about my work. This professor has been the most frustrating instructor I have ever had. I really appreciate your insights. Thanks! Mary

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