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### Re: Density Lab B

Posted: September 17th, 2018, 6:38 pm
Unome wrote:
Killboe wrote:
John Richardsim wrote:I don't exactly understand why they removed calculations of molarity from 3.d.ii. At first I thought that it was to avoid moles, but they left the ideal gas law in 3.d.iii, so I don't really know.

Another thing about the rules I don't understand is why they limit the use of parts-per notation to million and billion. For example, if I wanted to make a question about something at a concentration of 5 parts-per-trillion, couldn't I just say a concentration of 0.005 parts-per-billion in order to get around this?
'

I like how I spent like an hour studying how to calculate molarity and stuff like that and then I just see this comment. kmn

It's an extremely useful thing to know anyway if you're ever going to do any chem events or take any chem classes in the future.

### Re: Density Lab B

Posted: September 22nd, 2018, 7:37 am
Any of you know how to solve the number density of the green skittles in a bag thing?

### Re: Density Lab B

Posted: September 27th, 2018, 7:46 pm
I think the number density of skittles or M&Ms in a bag is kind of a weird question to ask. If the volume is just in units of "bags", then it's literally just counting the number of skittles/M&Ms of a certain color. If it is in units of cubic centimeters or the sorts, then there's the difficulty of being able to measure the volume of the bag, which would be able to change once the bag is opened. The only thing I can imagine for this would be dunking the bag in a beaker or large graduated cylinder to measure the volume and then opening the bag to count.

I also think having this in a test would raise certain logistical issues as well. If one bag is provided for all teams, skittles/M&Ms are bound to go missing. If a bag is provided for each team, the exact counts and measurements would vary between teams, making it pretty much impossible to evaluate teams on their accuracy.

Am I not thinking of something here?

### Re: Density Lab B

Posted: September 29th, 2018, 12:20 pm
Do any of you know if there are any practice tests for Density Lab?

### Re: Density Lab B

Posted: September 29th, 2018, 12:26 pm
someone1580 wrote:Do any of you know if there are any practice tests for Density Lab?

On the wiki if you go to the old test exchange, on the 2018 full test set for Garnet Valley Invitational, there should be a test for Density that was run as a trial last year (since its a trial test, it is fairly basic, so it would be a good place to start off with, but don't expect tests this year to be like that).

### Re: Density Lab B

Posted: October 4th, 2018, 8:17 pm
someone1580 wrote:Do any of you know if there are any practice tests for Density Lab?

syo_astro's SSSS test on the test exchange

(although there were rule changes between now and then)

### Re: Density Lab B

Posted: October 15th, 2018, 12:48 pm
Is this event math related?

### Re: Density Lab B

Posted: October 15th, 2018, 1:01 pm
Incineroar999 wrote:Is this event math related?

Yes. Very. Although nothing complicated, more like multiplication and division.

### Re: Density Lab B

Posted: October 15th, 2018, 2:20 pm
Are there certain tools or materials I need to know how to use for this event?

### Re: Density Lab B

Posted: October 15th, 2018, 6:02 pm
gl10086 wrote:Are there certain tools or materials I need to know how to use for this event?

Probably just know how to use a ruler or read a graduated cylinder.

### Re: Density Lab B

Posted: October 29th, 2018, 12:18 pm
Does anyone know what materials we need for the density lab?

### Re: Density Lab B

Posted: October 29th, 2018, 1:56 pm
Incineroar999 wrote:Does anyone know what materials we need for the density lab?

A binder, writing materials, and calculators

### Re: Density Lab B

Posted: October 29th, 2018, 3:58 pm
What is 1 mol of gas at STP equal too?

The wiki says that its 22.71 L. https://scioly.org/wiki/index.php/Densi ... ro.27s_Law
Simply looking it up, says its 22.4 L.

Which do I believe?

Thanks,
Bob

### Re: Density Lab B

Posted: October 29th, 2018, 6:01 pm
bobthebuilderman wrote:What is 1 mol of gas at STP equal too?

The wiki says that its 22.71 L. https://scioly.org/wiki/index.php/Densi ... ro.27s_Law
Simply looking it up, says its 22.4 L.

Which do I believe?

Thanks,
Bob

22.4 L / mol is the value for an ideal gas (calculated from PV=nRT). A calculation that better approximates a real gas, such as the van der Waals equation, may have a different value - I suspect that's where 22.71 comes from. In general, I would recommend using 22.4 since it's more commonly cited. see the next post

### Re: Density Lab B

Posted: October 29th, 2018, 8:04 pm
Unome wrote:
bobthebuilderman wrote:What is 1 mol of gas at STP equal too?

The wiki says that its 22.71 L. https://scioly.org/wiki/index.php/Densi ... ro.27s_Law
Simply looking it up, says its 22.4 L.

Which do I believe?

Thanks,
Bob

22.4 L / mol is the value for an ideal gas (calculated from PV=nRT). A calculation that better approximates a real gas, such as the van der Waals equation, may have a different value - I suspect that's where 22.71 comes from. In general, I would recommend using 22.4 since it's more commonly cited.

The 22.71 L/mol uses 1 bar for STP (used by IUPAC since 1982), whereas 22.41 L/mol uses 1.01325 kPa for STP (used before 1982).

If in doubt, be sure to state the specific conditions for STP that you are using.