## Chemistry Lab C

Skink
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### Re: Chemistry Lab C

Nerd_Bunny wrote:How is everyone studying for physical properties? Since this topic hasn't been in rotation for a while I'm having trouble finding any resources to help study.

This subtopic really has me thrown. Like, are we going as far as Ksp for solubility, or is it B division-grade conductivity measurements, for example? It's so hard to get a read for what they're going for with this when B division goes introductory high school while this event doesn't want to go AP level.

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### Re: Chemistry Lab C

I was trying to do slowpoke's SSS chem lab test, and I don't know how to do 2c of the thermodynamics open response. Can someone explain it for me? It says to calculate the standard enthalpy of formation for liquid benzene, given dH(f) for gaseous benzene is 82.9 kJ/mol.

Other givens are: dS for vaporization of benzene is -90.91 J/(mol*K), enthalpy of vaporization is -32.11 kJ/mol, its boiling point is 353.2 K, and exhibits a vapor pressure of .526 atm at 333.6 K.

Thanks so much!
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### Re: Chemistry Lab C

Nerd_bunny wrote:How is everyone studying for physical properties? Since this topic hasn't been in rotation for a while I'm having trouble finding any resources to help study.

As a test writer, I focused more on the concepts behind the physical properties than on the properties themselves. For instance, I tested the ideas behind say, why some things are denser than others or how dissolving things in water works. The only issues is that while that approach works well for like conductivity (you need to understand intermolecular forces), things like color require pulling material from organic or even undergrad level inorganic chemistry. Magnetism and solubility are also super vague. It's pretty hard to figure out how far as a test writer you're supposed to go, or what SciOly intended exactly with their rules.

The questions I attempted to avoid were memorization problems. For instance, I wouldn't put a "Order the density of these metals" thing on the exam because 1) it requires you to dedicate a not insignificant space to record information that doesn't really help you progress as a chemist and 2) it's better to test say, the reasons why some metals are denser (ie: they have a more highly packed structure).

All in all, I dislike this topic somewhat intensely. It's hard to find a physical properties lab that doesn't take over 50 minutes or require extensive equipment and preparation. I have not seen a single competition that did not do a thermo lab, and I'm sure that when I do, it will be a highly janky lab. Also, the list of properties feels super arbitrary. Why elasticity, but not hardness? Does elasticity include ductility and malleability also? This topic could have been my favorite if it were more specific, but it's way too vague.

Tl;dr: focus on causes of physical properties and understanding what those properties exactly are.
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### Re: Chemistry Lab C

This is my first year doing science Olympiad and I cannot fully understand the wiki for Chem Lab, my event is this Saturday, are there any things I should add to my sheet that people tend to forget?

geniusjohn5
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### Re: Chemistry Lab C

Are there any good resources/links to prepare for the Lab portion of this event?
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### Re: Chemistry Lab C

Does anyone know what reference sheets the test proctors will give to us test-takers? For example, will a Periodic Table be provided at the test?
The physicist says, "In the universe, does matter really matter?"
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kenniky
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### Re: Chemistry Lab C

geniusjohn5 wrote:Does anyone know what reference sheets the test proctors will give to us test-takers? For example, will a Periodic Table be provided at the test?

Proctors are supposed to provide all constants, which would include a periodic table (or at least relevant molar masses)

Most tests I have seen do provide these, but sometimes they don't. If you have extra space on your cheat sheet, I would highly recommend adding a periodic table, a list of common enthalpies of formation and specific heats, and other important constants in case the ES doesn't read the rules carefully enough.
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### Re: Chemistry Lab C

Chem Lab is allowed to have 5 pages for cheat sheets, but I have no idea how to fill up all the space.
Did anyone completely fill theirs, or does anyone have any ideas for what to add?

kenniky
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### Re: Chemistry Lab C

huppada wrote:Chem Lab is allowed to have 5 pages for cheat sheets, but I have no idea how to fill up all the space.
Did anyone completely fill theirs, or does anyone have any ideas for what to add?

Lists of enthalpies, entropies, Gibbs energies, colors

Common constants (ex: anything to do with water)

If you can fit it onto 2 sheets you can make a copy for you and your partner.

I do agree that 5 is overkill for Chem Lab
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Nerd_Bunny
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### Re: Chemistry Lab C

kenniky wrote:
huppada wrote:Chem Lab is allowed to have 5 pages for cheat sheets, but I have no idea how to fill up all the space.
Did anyone completely fill theirs, or does anyone have any ideas for what to add?

Lists of enthalpies, entropies, Gibbs energies, colors

Common constants (ex: anything to do with water)

If you can fit it onto 2 sheets you can make a copy for you and your partner.

I do agree that 5 is overkill for Chem Lab

My partner and I are completely taking advantage of those 5 sheets. Anything we come across, we put on the sheet. We have pretty large font yet still have tons of room.We're also both doing A&P, so we know how much extra sheets can help. I'd recommend adding as many images, charts, and flowcharts as possible. We have an electron configuration chart, (two actually) a phase diagram, an electronegativity chart, and room for a full periodic table. Don't forget to put some common densities and specific heat values on there too. Event supervisors are supposed to give you these values but sometimes they don't. Really take advantage of those 5 pages.
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### Re: Chemistry Lab C

Nerd_Bunny wrote:
kenniky wrote:
huppada wrote:Chem Lab is allowed to have 5 pages for cheat sheets, but I have no idea how to fill up all the space.
Did anyone completely fill theirs, or does anyone have any ideas for what to add?

Lists of enthalpies, entropies, Gibbs energies, colors

Common constants (ex: anything to do with water)

If you can fit it onto 2 sheets you can make a copy for you and your partner.

I do agree that 5 is overkill for Chem Lab

My partner and I are completely taking advantage of those 5 sheets. Anything we come across, we put on the sheet. We have pretty large font yet still have tons of room.We're also both doing A&P, so we know how much extra sheets can help. I'd recommend adding as many images, charts, and flowcharts as possible. We have an electron configuration chart, (two actually) a phase diagram, an electronegativity chart, and room for a full periodic table. Don't forget to put some common densities and specific heat values on there too. Event supervisors are supposed to give you these values but sometimes they don't. Really take advantage of those 5 pages.

I am also in A&P, and I agree, extra pages really help. But does it get difficult to locate things at times? 5 sheets is really 10 pages, and it seems like a hassle to have to search through those pages in the middle of a competition.

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### Re: Chemistry Lab C

Nerd_Bunny wrote:
kenniky wrote:Lists of enthalpies, entropies, Gibbs energies, colors

Common constants (ex: anything to do with water)

If you can fit it onto 2 sheets you can make a copy for you and your partner.

I do agree that 5 is overkill for Chem Lab

My partner and I are completely taking advantage of those 5 sheets. Anything we come across, we put on the sheet. We have pretty large font yet still have tons of room.We're also both doing A&P, so we know how much extra sheets can help. I'd recommend adding as many images, charts, and flowcharts as possible. We have an electron configuration chart, (two actually) a phase diagram, an electronegativity chart, and room for a full periodic table. Don't forget to put some common densities and specific heat values on there too. Event supervisors are supposed to give you these values but sometimes they don't. Really take advantage of those 5 pages.

I am also in A&P, and I agree, extra pages really help. But does it get difficult to locate things at times? 5 sheets is really 10 pages, and it seems like a hassle to have to search through those pages in the middle of a competition.

It hardly takes my partner and I much time to find what we need. All of our vocab is alphabetical, and that takes up about 3 pages. The rest are large images, so usually it takes less than 20 seconds to find something.
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geniusjohn5
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### Re: Chemistry Lab C

Can anyone suggest what I should put on my cheat sheet for PHYSICAL PROPERTIES?
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### Re: Chemistry Lab C

Has anyone taken the University of Michigan invite test? My partner and I did the test yesterday and we were wondering if anyone was able to correctly do the math on the lab. Advice as well would be really helpful. Thanks!
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kenniky
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### Re: Chemistry Lab C

Nerd_Bunny wrote:Has anyone taken the University of Michigan invite test? My partner and I did the test yesterday and we were wondering if anyone was able to correctly do the math on the lab. Advice as well would be really helpful. Thanks!

Test writer here-

So let's say you have $x$ moles of NaCl and $y$ moles of CaCl2; we want to find $x$ and $y$ .

From the experiment you should get the mass $m$ of the sample, the mass of water used $m_w$ , the boiling temperature without dissolved salt $T_x$ , and the boiling temperature with salt $T_s$ .

The boiling point elevation formula says that $\Delta T_b=K_b mi$ where $m$ is the molality and $i$ is the Van't Hoff factor (sp?), which can be approximated as the number of ions that one molecule of a substance dissociates into. In particular, $i_\text{NaCl}=2$ and $i_{\text{CaCl}_2}=3$ .

Here, $\Delta T_b=T_s-T_x$ . Since the two salts have different Van't Hoff factors, their contributions need to be calculated separately; $\Delta T_b=\Delta T_{b,\text{NaCl}}+\Delta T_{b,\text{CaCl}_2}$ , $\Delta T_{b,\text{NaCl}}=K_b m_{\text{NaCl}}i_\text{NaCl}$ and $\Delta T_{b,\text{CaCl}_2}=K_b m_{\text{CaCl}_2}i_{\text{CaCl}_2}$ . Since we assumed $x$ moles of NaCl and $y$ moles of CaCl2, $m_\text{NaCl}=\frac{x}{m_w}$ and $m_{\text{CaCl}_2}=\frac{y}{m_w}$ . So you end up with $T_s-T_x=\frac{K_b i_\text{NaCl}}{m_w} x + \frac{K_b i_{\text{CaCl}_2}}{m_w} y$ , which looks kind of ugly but is just an equation in $x$ and $y$ since everything else is known.

Also, from the masses you get $m=m_{\text{NaCl}}+m_{\text{CaCl}_2}$ , note that the $m$ s here are mass not molality (why does chemistry use the same symbols for everything qq). So $m={MM}_\text{NaCl} x+{MM}_{\text{CaCl}_2} y$ which is also an equation in $x$ and $y$ .

So you just have a system of equations:

$T_s-T_x=\frac{K_b i_\text{NaCl}}{m_w} x + \frac{K_b i_{\text{CaCl}_2}}{m_w} y$
$m={MM}_\text{NaCl} x+{MM}_{\text{CaCl}_2} y$

which should be solvable, making sure to account for SI units and such.

For example, if $T_x=100$ C, $T_s=103.469$ C, $m_w=0.1$ kg, and $m=24$ g, you have

$103.469-100=\frac{0.512* 2}{0.1} x + \frac{0.512* 3}{0.1} y$
$24=58.44 x+110.98 y$

from which you get $x=0.0684$ moles and $y=0.18$ moles and then you can find the percent mass from there

In retrospect I probably should have given a procedure for everyone to follow at least, so that they could get to the math here because very few people ended up even considering $\Delta T_b=K_b mi$ , or just made it a test question and had a slightly easier lab ... oops

Feel free to ask me other questions about my test if my messy typed out equations didn't make sense lol
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