General non-science questions and class announcements.

Chem_Mod
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The title is called "Pizza Rolls" so that you can search "Pizza Rolls" and easily find this post!

This is your favorite UA, Lyndon Bui, and Matt and I are proud to present practice problems for you midterm YAY. The questions are modeled after previous test and exam questions, but it does not have everything you need to know. This covers material for the midterm.

Please complete these problems before the review session. The review session this quarter (W2020) will be Sunday, Feb 9, 2-5pm, in Moore 100.
We will go over each problem and concept in detail during the review session. The answers, but not full solutions or work, will be posted here at this post after the review session. Any errors in the test will be posted below as well.

Printed copies will NOT be provided at the review session.

PizzaROLLSSSS.pdf

***This is not indicative of the structure, length, or format of the actual final. Treat these as extra practice problems. ***

Errors: None Found Yet

Will be posted after the review session.

Happy Studying and Good Luck!
-Lyndon Bui, UA

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### Re: Pizza Rolls REVIEW Session

Thank you so much!! Will you be providing another packet or will you go over this packet on your Monday workshop?

ELu 1J
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### Re: Pizza Rolls REVIEW Session

Are you only going over the answers at the Sunday review session or the Monday one as well? Thank you so much :)))

SarahCoufal_1k
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### Re: Pizza Rolls REVIEW Session

ELu 1J wrote:Are you only going over the answers at the Sunday review session or the Monday one as well? Thank you so much :)))

they usually go over it in the three hour session on Sunday only

ELu 1J
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Is the Sunday review session only for going over the practice questions and then the one on Monday is content-based review?

Chem_Mod
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ELu 1J wrote:Is the Sunday review session only for going over the practice questions and then the one on Monday is content-based review?

The Monday Review session was moved to Sunday. There is only a Sunday one. On Monday, Lyndon does have normal UA workshop hours but it is not a review session.

Sunday's session will cover concepts and the practice questions both.

chimerila
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Thanks so much! Love the little funnies throughout the problem sets. If we have a good understanding of the concepts on this practice exam, do you think we're at a good place for the test?

Yailin Romo 4G
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Thanks so much I will be attending tomorrow!

Naren_Ramesh_4F
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Will the answer key be posted on Chemistry Community?

DesireBrown1J
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What was the little saying to remember the state functions other than entropy and enthalpy? The HUGS and TV one?

Sofia Barker 2C
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DesireBrown1J wrote:What was the little saying to remember the state functions other than entropy and enthalpy? The HUGS and TV one?

To remember the state functions, the saying was something along the lines of "if you're feeling under Pressure or Depressed, go watch some TV or get HUGS." So the P (pressure) in pressure, the D (density) in depressed, T (temp), V (volume), H (enthalpy), U (internal energy), G (Gibbs free energy), and S (entropy) are all state functions.

AysiaB1I
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Thank you!!

Mariana Fuentes 1L
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Thank you so much!!!

DesireBrown1J
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Why is delta H always q but q is not always delta H? What exactly does he mean by that?

Leyna Dang 2H
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DesireBrown1J wrote:Why is delta H always q but q is not always delta H? What exactly does he mean by that?

DeltaH by definition is q at constant pressure. But if q is not at constant pressure then it is not deltaH.

Leyna Dang 2H
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DesireBrown1J wrote:What was the little saying to remember the state functions other than entropy and enthalpy? The HUGS and TV one?

If you’re under Pressure and feeling Dense, all you want is to watch TV and get HUGS.

005206171
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DesireBrown1J wrote:Why is delta H always q but q is not always delta H? What exactly does he mean by that?

The definition of enthalpy is energy/heat absorbed or released at constant pressure (state function). So delta H is always q. Heat only describes energy transfer due to a temperature difference. Heat can be exchanged even when pressure isn’t constant. For example in isochoric conditions, volume isn’t changing so no work can be done by the system. Since delta U = q + w and w=0, q = delta U. Hope this helps

Johnathan Smith 1D
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When will the answers be posted for the review questions?

Jamie Hwang 2F
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thank you!

Ariana Iranmahboub1G
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Can someone show me step by step on how to solve for number 10. I did not understand why q(ice)=q(water).

CHEM MOD: q(ice) = -q(water) DO NOT forget the negative sign. The heat released from one material in a system is absorbed by the other material in the system.

Ian Morris 3C
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Naren_Ramesh_4F wrote:Will the answer key be posted on Chemistry Community?

Yes they are usually but after the review session.

MAC 4G
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Does anyone know when the answer key will be posted?

Chem_Mod
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MAC 4G wrote:Does anyone know when the answer key will be posted?

Sally Qiu 2E
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can someone explain the lex luthor problem to me? it's number 5. i think for part a, i need to use the 2 entropy equations as well as the equipartition thing. and i'm not sure about b...

Emily Lo 1J
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Can someone help explain why and how the equation for 3b includes (1/2)(m)(delta H of fusion)? I get what you would isolate to get the answer but I'm not sure how they got that part of the equation.

205150314
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Can someone help me solve problem 3B? I wasnt able to make it to the review session and the enthalpy of fusion is causing me problems

Hailey Kim 4G
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Ariana Iranmahboub1G wrote:Can someone show me step by step on how to solve for number 10. I did not understand why q(ice)=q(water).

CHEM MOD: q(ice) = -q(water) DO NOT forget the negative sign. The heat released from one material in a system is absorbed by the other material in the system.

For #10, why do we have to convert the 25.0 g of ice into moles for the phase change equation? (Why do we use n(delta H) instead of m(delta H))

Hailey Kim 4G
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Emily Lo 1J wrote:Can someone help explain why and how the equation for 3b includes (1/2)(m)(delta H of fusion)? I get what you would isolate to get the answer but I'm not sure how they got that part of the equation.

For problem 3B, you have to use the equation q=mC(Delta T) but also the phase change equation q=m(delta H of fusion). However, the phase change equation is multiplied by one half because the problem says that only HALF of the ice cream melted.

ShastaB4C
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What did I do wrong? The answer is supposed to be 16 degrees Celsius
Attachments

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ShastaB4C wrote:What did I do wrong? The answer is supposed to be 16 degrees Celsius

The ice goes through a phase change so you need to add that heat to the equation on the side of the ice.

JosephineF
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as long as our units are consistent does it matter if we use kJ or J when talking about enthalpy or entropy?

JosephineF
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what is the difference between extensive and intensive properties?

EMurphy_2L
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Sally Qiu 2E wrote:can someone explain the lex luthor problem to me? it's number 5. i think for part a, i need to use the 2 entropy equations as well as the equipartition thing. and i'm not sure about b...

Yes for part A you need to consider the entropy change in 3 parts: change in Kr, change in He, and change in temp. using Hess's law you add all together to get total entropy

for both Kr and He there is a volume change so you use S = nR ln(V2/V1)
*the key is that the V2/V1 will be 4 for He and 4/3 for Kr. although first insticnt may tell you it's a 3:1 ratio, if you plug in values like 1 and 3 you will see that V2 is 4!!

for the tamp change you use S = nC ln(T2/T1)
*C will be 3/2R because it's a monoatomic ideal gas!

EMurphy_2L
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JosephineF wrote:what is the difference between extensive and intensive properties?

an extensive property is one based on the amount of substance. intensive is one that is general, despite amount.

heat capacity is extensive because you need to know the number of moles or grams.
specific heat capacity is intensive because it is PER moles or grams.

EMurphy_2L
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205150314 wrote:Can someone help me solve problem 3B? I wasnt able to make it to the review session and the enthalpy of fusion is causing me problems

Hi! someone answered a similar question above but for 3B you are given more info than you think. First you need to realize that the ice cream both warms in solid phase and then goes through HALF of a phase transition. if you think of the phase change graph, q is the x-axis and always increasing so you can add these two processes to get total q.
q = nCΔT or mCΔT is for a substance in phase (use C from part A!!)
q = nΔH or mΔH is for a substance in phase change (not on constants sheet) BUT since only half melts you use 1/2mΔT
therefore the total equation is q = mCΔT + mΔH (use m equation for both because you're given grams) and you just solve for ΔH because that's what they're asking for!!

JosephineF
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for number 7 I understand how to solve for q but how do you solve for w with so many missing variables from the pv=nrt equation?

zfinn
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can someone please explain #5 i am very confused. thank you

Amy Xiao 1I
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3 E and 3 F

Can someone explain why these two statements are false? I don't really understand the wording of 3E, and is it ever possible to have a negative heat capacity?

3E: Removing heat from a system is exothermic, so temperature must decrease.

3F: Heat Capacities can be negative for simple molecules.

Janet Nguy 2C
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For question 10 (25.0g ice at 0.00 Celcius dropped into 265 mL of water at 25.0 Celcius. What is Tf of the water?):

Why can we assume that all 25 grams of ice melted into water? i.e. why is the m1 (mass for the ice undergoing phase change) equal to the m2 (mass of the ice that becomes water)? How do we know that all of it melted?

Janet Nguy 2C
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JosephineF wrote:for number 7 I understand how to solve for q but how do you solve for w with so many missing variables from the pv=nrt equation?

The general equation for work is $w=-p\Delta V$ at a constant pressure. Because we are given the $\Delta H$ value, we can assume that the reaction occurred under constant pressure (because $\small \Delta H = q_{p}$).

Thus, using the ideal gas equation (pV = nRT), we can rearrange the values such that it matches the work equation. In this case, we arrange to write $\small p\Delta V = \Delta nRT$, because in this case we have 3 moles of gas being converted into 5 moles of gas in the reaction.
We can now substitute this in for the work equation to get $\small w = -\Delta nRT$, where $\small \Delta n$ is equal to the moles of gas in the product minus the moles of gas on the reactant side (5 moles - 3 moles = 2 moles).

Now, we can solve for $\small \Delta U$. We know $\small \Delta n$, R, and T (room temperature is 298.15 K). This value, however, is the change in internal energy for the stated reaction.

To find $\small \Delta U$ for 1.00 mol of CO2, divide by 4. That should yield $\small \Delta U = -62.5 kJ$.

Hope this helps!

zfinn
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For Pizza Rolls 5e why is the answer false? is it only crystals that have 0 entropy at absolute zero?

Asha Agarwal 1E
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Can someone explain how to solve #11 on the practice questions? What steps do you need to take?

Asha Agarwal 1E
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Can someone also explain #12B? Is there a specific equation to use?

Julia Holsinger_1A
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I am confused with #12B. What equation do you use when trying to find delta H?

Zoya Mulji 1K
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Hailey Kim 4G wrote:
Ariana Iranmahboub1G wrote:Can someone show me step by step on how to solve for number 10. I did not understand why q(ice)=q(water).

CHEM MOD: q(ice) = -q(water) DO NOT forget the negative sign. The heat released from one material in a system is absorbed by the other material in the system.

For #10, why do we have to convert the 25.0 g of ice into moles for the phase change equation? (Why do we use n(delta H) instead of m(delta H))

The deltaH fusion given in the constants list is in kJ/mol so you have to convert the 25.0 grams of ice to mol to go with the units on the deltaH of fusion.

ShastaB4C
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ShastaB4C wrote:What did I do wrong? The answer is supposed to be 16 degrees Celsius

The ice goes through a phase change so you need to add that heat to the equation on the side of the ice.

Do I get that value from delta H fusion on the equation sheet?

Luyan Zhang - 2D
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Can someone explain #6 to me? I dont really understand how to calculate each one of them. And is delta U = 0?

Julia Holsinger_1A
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Asha Agarwal 1E wrote:Can someone also explain #12B? Is there a specific equation to use?

You take the q=mC(delta)T of each part of the equation while using the Cpm values given for C and then you do the sum of the products minus the sum of the reactants.

Luc Zelissen 1K
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My final equation for number 10 is $(25_{grams}*2.01_{C_{Ice} }*T_{f}-0) + (6010_{\Delta Hfus_{Jules}}*1.38_{moles})= -(265_{grams}*4.184_{C_{Liquid}}*(T_{f}-25))$, but I keep getting the wrong degrees. What am I doing wrong?

Luc Zelissen 1K
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For Question 5, I have $\Delta S=moles*8.314*ln(v2/v1)$ for both gasses, but when I add them up I dont get the right answer. Is there another equation to find the change of Entropy of the 2 gasses combined, if so what is it?

Vuong_2F
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Asha Agarwal 1E wrote:Can someone explain how to solve #11 on the practice questions? What steps do you need to take?

1) since you're dealing with an acid, you want to convert the pKb to Ka
2) given the concentration of the acid, you can set up an ice table to help you write the equilibrium expression
3) after setting up the equilibrium expression, solve for x. x is also the value of the H+ concentration
4) use -log[H+] to find the pH

Jocelyn Thorp 1A
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Luc Zelissen 1K wrote:For Question 5, I have $\Delta S=moles*8.314*ln(v2/v1)$ for both gasses, but when I add them up I dont get the right answer. Is there another equation to find the change of Entropy of the 2 gasses combined, if so what is it?

don't forget to account for the temperature change!

Luc Zelissen 1K
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Luc Zelissen 1K wrote:My final equation for number 10 is $(25_{grams}*2.01_{C_{Ice} }*T_{f}-0) + (6010_{\Delta Hfus_{Jules}}*1.38_{moles})= -(265_{grams}*4.184_{C_{Liquid}}*(T_{f}-25))$, but I keep getting the wrong degrees. What am I doing wrong?

I realized that after the dHfus occurs, the ice turns to liquid, so you need to use the specific heat capacity of water on both sides, so it should look like $(25_{grams}*4.184_{C_{liquid} }*T_{f}-0) + (6010_{\Delta Hfus_{Jules}}*1.38_{moles})= -(265_{grams}*4.184_{C_{Liquid}}*(T_{f}-25))$

EMurphy_2L
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HELP for question 6, why is ΔU=0 and ΔS=0 for all three changes?

Dan M -3E
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Can someone pls explain how to find heat and work in question #6? I get that deltaU and deltaS are 0 because they are state functions, but how/what equations do I use to find work and heat?

Jocelyn Thorp 1A
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Vuong_2F wrote:
Asha Agarwal 1E wrote:Can someone explain how to solve #11 on the practice questions? What steps do you need to take?

1) since you're dealing with an acid, you want to convert the pKb to Ka
2) given the concentration of the acid, you can set up an ice table to help you write the equilibrium expression
3) after setting up the equilibrium expression, solve for x. x is also the value of the H+ concentration
4) use -log[H+] to find the pH

could someone walk me through this with the actual values? I tried these steps, but end up getting a wrong answer :(
or at the very least, could someone write what they got as equal to the Ka value (in terms of x) after setting up the ice table?

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EMurphy_2L wrote:
205150314 wrote:Can someone help me solve problem 3B? I wasnt able to make it to the review session and the enthalpy of fusion is causing me problems

Hi! someone answered a similar question above but for 3B you are given more info than you think. First you need to realize that the ice cream both warms in solid phase and then goes through HALF of a phase transition. if you think of the phase change graph, q is the x-axis and always increasing so you can add these two processes to get total q.
q = nCΔT or mCΔT is for a substance in phase (use C from part A!!)
q = nΔH or mΔH is for a substance in phase change (not on constants sheet) BUT since only half melts you use 1/2mΔT
therefore the total equation is q = mCΔT + mΔH (use m equation for both because you're given grams) and you just solve for ΔH because that's what they're asking for!!

Hey for $q=m\Delta H$, you multiplied by 1/2 but your eq. says $1/2m\Delta T$. Is this just a typo or am I missing something?

Clara Cho 2K
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Can someone please explain why the v2/v1 for He is 4 and and 4/3 for Kr for problem 5?

Jocelyn Thorp 1A
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Clara Cho 2K wrote:Can someone please explain why the v2/v1 for He is 4 and and 4/3 for Kr for problem 5?

Yeah so V2 is the sum of the volume in both chambers, where one is 3 times the size of the other. So in one chamer you have one liter, and the other you have 3. So the total (V2) is 4. He starts out in the 1L container so V2/V1 is 4/1 and Kr starts out in the 3L chamber so V2/V1 is 4/3. hope this helped!

Clara Cho 2K
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I feel like I'm doing #6 correctly , but I'm not getting the correct answer. Can someone explain how I calculate the work of expansion for the step with the reversible, isothermal expansion?

Jared Khoo 1G
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Clara Cho 2K wrote:I feel like I'm doing #6 correctly , but I'm not getting the correct answer. Can someone explain how I calculate the work of expansion for the step with the reversible, isothermal expansion?

Wby = nRT (ln V2/V1), and -Wby = Won which is the component of dU = q + w.

kennedyp
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Can someone explain 2 D? For some reason I'm not getting the right answer for the reaction quotient :(

CalvinTNguyen2D
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kennedyp wrote:Can someone explain 2 D? For some reason I'm not getting the right answer for the reaction quotient :(

For 2D, remember that NH4HS(s) is not included in the reaction quotient, since it is a solid! The equation should just be Q = [H2S][NH3]! Make sure to convert the mols into MOLAR by dividing the mols by 2L. I hope this helps :)

AysiaB1I
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Does anyone know how we got the temperature for question 6 on pizza rolls? I have 304.55 from the review session but i'm not sure how we got it.

Jocelyn Thorp 1A
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Joined: Wed Sep 18, 2019 12:20 am

AysiaB1I wrote:Does anyone know how we got the temperature for question 6 on pizza rolls? I have 304.55 from the review session but i'm not sure how we got it.

solve for it with PV=nRT, everything except temp is given in the problem

Dan M -3E
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Thanks to everyone that helped out with questions that people were asking here. It was a big help for reviewing for the midterm.

DesireBrown1J
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Joined: Wed Sep 18, 2019 12:18 am

Does Lyndon have a practice worksheet for Test 2?

Diana Chavez-Carrillo 2L
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