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 Sammy Sep 14th 2018 10:51 AM

Lab Question

I have to do a chem prelab for this week and two of the questions ask for specific reasons behind the procedure and method. Part of the lab procedure is to find the mass and volume of three glass rods- one small one, and two large ones with different lengths. Another step is to find the mass and volume of the two larger rods together. The entire lab is about finding a relationship between mass and volume so it's obvious why I have to find the individual masses but I'm stuck on why the mass and volume of the combined rods are important. Is it just to see if the relationship for the individual rods is constant even when applied to combined items?

Also would it be necessary to put down finding the combined mass/volume as a separate step or would it fall under the same step as finding the masses of the rods?

 Sammy Sep 14th 2018 11:26 AM

Turns out we also gotta find the mass of a beaker/watch glass. Just the mass, not the volume. What would be the reason behind that? It doesn't seem to figure into any other part of the lab

 Woody Sep 17th 2018 12:20 AM

Sometimes it seems that tutors are just making it up as they go...
You can sometimes guess the reasoning by looking at the general trajectory indicated from previous lessons,
but not from just by looking at the isolated problem.

Have the methods for finding the mass and volume been specified?

 Sammy Sep 17th 2018 03:51 AM

Quote:
 Originally Posted by Woody (Post 153480) Sometimes it seems that tutors are just making it up as they go... You can sometimes guess the reasoning by looking at the general trajectory indicated from previous lessons, but not from just by looking at the isolated problem. Have the methods for finding the mass and volume been specified?
Yes, we find the mass by just putting the rods on a balance and the volume by sticking them in water and subtracting the waters volume from the total. This is due tomorrow and Im still really confused about it :(

 Woody Sep 17th 2018 04:59 AM

Dont over think it...

Can one assume that the mass to volume relationship (density) of the glass rods will be identical?
It looks to me that the tutor is suggesting a totally pedantic methodology, making as few prior assumptions as possible.

I suspect that you are going to be expected to derive the definition of density from a series of observations,
all the while making no assumptions about you having any prior knowledge about density.

Some of your class colleagues might actually have such a knowledge gap,
just because you expect to see a known relationship between mass and volume,
your tutor cannot assume that everyone in the group has such prior knowledge.

 Sammy Sep 17th 2018 06:03 AM

Quote:
 Originally Posted by Woody (Post 153482) Can one assume that the mass to volume relationship (density) of the glass rods will be identical? It looks to me that the tutor is suggesting a totally pedantic methodology, making as few prior assumptions as possible. I suspect that you are going to be expected to derive the definition of density from a series of observations, all the while making no assumptions about you having any prior knowledge about density. Some of your class colleagues might actually have such a knowledge gap, just because you expect to see a known relationship between mass and volume, your tutor cannot assume that everyone in the group has such prior knowledge.

Yeah, I figured itll involve density at some point, but we have to do this from looking at the actual lab protocol which doesnt bring up density....I'm just having trouble with this because the assignment keeps saying to be specific with the steps and reasons but to me some of it is confusing without actually doing the lab first. Plus my chem professor is that wonderful combination of really strict without being any real help. We never went over what exactly was expected in prelabs so I'm trying to err on the side of too much information, should I not be doing that?

 Woody Sep 17th 2018 06:27 AM

It is difficult to advise,
It depends too much on (unknown) information regarding your tutors expectations.

I think you should be looking for practical issues.
What sources of error can you foresee, how can you minimise them?
Can you physically fit the rods onto the weighing machine?
(might they roll about awkwardly?)
Can you fully submerge the rods in the water and measure the volume change?
(what is your volume measuring mechanism?)
How accurately will you be able to determine the mass?
How accurately will you be able to determine the volume?
You might want to consider repeating the tests, to ensure the results are reliably reproducible.

You could perhaps include a mention of pre-lab expectations
possibly with criteria for accepting or rejecting the hypothesis,
but that seems a bit advanced (compared with the basic simplicity of the experiment).

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