Chemistry Lab C

huppada · Post by **huppada** » Sun Feb 25, 2018 1:27 pm

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.

Nerd_Bunny · Post by **Nerd_Bunny** » Sun Feb 25, 2018 1:34 pm

huppada wrote:
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.

geniusjohn5 · Post by **geniusjohn5** » Thu Mar 01, 2018 12:48 pm

Can anyone suggest what I should put on my cheat sheet for PHYSICAL PROPERTIES?

Nerd_Bunny · Post by **Nerd_Bunny** » Wed Mar 21, 2018 9:43 am

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!

kenniky · Post by **kenniky** » Wed Mar 21, 2018 1:45 pm

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

Nerd_Bunny · Post by **Nerd_Bunny** » Wed Mar 21, 2018 2:22 pm

kenniky wrote:
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

Thank you! This helps so much! My partner will be very pleased to see this.

Nerd_Bunny · Post by **Nerd_Bunny** » Thu Mar 29, 2018 3:20 pm

Would the electromagnetic spectrum come up on a test? I don't think it would, but has anyone had a test question on it?

Post by **raxu** » Sat Apr 14, 2018 9:51 am

Hi! I was practicing Chem Lab and for a few calculations I didn't agree with the answer key. Can you check my work? Thanks!

1. An object with mass 46.2g displaces 0.34 Liters of water when submerged in a tank. What is the object’s density?
46.2g/340cm^3=0.135g/cm^3.

2. Lead has a melting point of 327.5 °C, specific heat 0.128 J/g ⋅°C, and molar enthalpy of fusion 4.80 kJ/mol. How much heat, in kJ, will be required to heat a 500.0 g sample of lead from 23.0 °C to its melting point and then melt it?
500g*0.128J/gC*304.5C+500g/(207.2g/mol)*4800J/mol=31kJ.

Thanks again!

Excelsior · Post by **Excelsior** » Wed May 09, 2018 8:26 pm

If tryouts go well, I will be entering Division C to compete for the next school year. I currently do Potions and Poisons in Division B and I was wondering, are there any concepts that transfer over? I know the topics rotate for Chem Lab but for Potions, everything is just set in whatever the rules are, the only differences being the toxic plants/animals tested. What are the similarities in the two events, if any?

Thanks!

Post by **UTF-8 U+6211 U+662F** » Fri May 11, 2018 6:14 pm

raxu wrote:Hi! I was practicing Chem Lab and for a few calculations I didn't agree with the answer key. Can you check my work? Thanks!

1. An object with mass 46.2g displaces 0.34 Liters of water when submerged in a tank. What is the object’s density?
46.2g/340cm^3=0.135g/cm^3.

2. Lead has a melting point of 327.5 °C, specific heat 0.128 J/g ⋅°C, and molar enthalpy of fusion 4.80 kJ/mol. How much heat, in kJ, will be required to heat a 500.0 g sample of lead from 23.0 °C to its melting point and then melt it?
500g*0.128J/gC*304.5C+500g/(207.2g/mol)*4800J/mol=31kJ.

Thanks again!

Both look good to me! (Although the density for 1 rounds to 0.136 g/cm^3)

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