hi so im currently on unit 7 of AP chem and i need some help understanding this unit—what specific formulas/equations do i need to know? (my AP chem teacher doesn’t teach so it’s difficult to understand some concepts in the class)

We have, in a volume of 0.1L

0.03 mol of BaCl₂

0.07 mol of Na₂SO₄

What is the concentration of Ba^(2)+ at equilibrium?

Is given : pKsp(BaSO₄) =10

The answer is 2.5*10^-7 M

I really don't know where to start with this one. I know [Ba^(2)+]*[SO₄]=10^-10

I don't know how to find [SO4]. All I can think of is that it should be equal to 0.5*[Na^(+)].

I thought maybe that NaCl would be combine as much as possible, leaving a certain amount of Ba^(2+) (0.03mol) and of SO₄ (0.07-0.04=0.03mol) floating around but that doesn't make much sense in water since NaCl should just dissolve...

I'd love some guidance for the next step. Also, is there a way to know when you can assume that a certain molecule will totally dissolve?

Thank you very much!

I've been working through UEarth and came across these two solubility equilibrium equations in two separate questions. After watching a Khan academy video, I know why Cl is multiplied by 2 in addition to having 2 as the exponent. But i'm not sure why we don't do the same for the second equation with OH?

This is my answer so far but I'm not sure if I'm right.

Edit:

This is my new answer pls help

the question is:

What is the molar solubility of Zn(OH)2 (Ksp = 4.1 × 10−17) in a 0.1 M solution of NaOH?

I understand the question and how to solve it and there is a solution to it in the back, however the book also states that any Hydroxide is not soluble in solution except with some exceptions (Zn is not an exception). Am I wrong that this would not dissociate as it is insoluble?

EDIT:formatting

My question is not really "homework." Im just wondering why in solubility problems we let the reaction go to completion before setting up the equilibrium problem. For example in this problem

Calculate the final concentrations of K1(aq), C2O4 22(aq), Ba21(aq), and Br2(aq) in a solution prepared by adding 0.100 L of 0.200 M K2C2O4 to 0.150 L of 0.250 M BaBr2. (For BaC2O4, Ksp = 2.3*10^-8)

the textbook solves it by first letting the reaction go to completion and then setting up the equilibrium problem. https://imgur.com/a/US91k However can't you just set up the equilibrium problem from the beginning? I envision it like a time line. https://imgur.com/a/2Kutf. The textbook jumps to point B, a little ahead of the answer (because it assumes complete precipitation) and as a result has to go a bit back to get to the answer by setting up an equilibrium problem (some of the precipitant dissolves). But couldn't you just set up the equilibrium from the start and let it take you to point A? Since point A is where equilibrium is.

The question without parameters for context:
In a solution of 2 soluble salts(M*gCl2* and ZnCl*2* with a given molarity, NaOH is added to bind with the cations (both product salt formulas are X(OH)2) and isolate the chloride. the question asks if the ions can be isolated to 99.9%. my questions are:

1. it is given that the NaOH solution being added is highly concentrated and no concentration is given - what am I supposed to plug into the ksp formula then for OH molarity?
2. what does it mean to reach a selective separation of 99.9%? does it mean that the weighted average of cations in the solution was reduced by this much or that the most prevalent cation was reduced by that much?
   TIA

The question is as follows

The equilibrium constant for the reaction below is 0.18 at 25 oC. PCl3(g) + Cl2(g)PCl5(g)

The following concentrations were measured from the reaction vessel:

[PCl3(g)] = 0.0420

[Cl2(g)] = 0.0240

[PCl5(g)] = 0.00500

Is this system in equilibrium? Explain. If not, in which direction will the system shift?

I did PCl5/(cl2)(Pcl3) and subbed in the values and got 4.96. This would mean the system is shifting towards the reactants and is not in equilibrium. Did I do this right? The answer seems much higher than the constant given IMO. If I didnt do it right, can someone point me in the right direction please?

Okay, so I'm a tad confused on one of the Equilibrium problems in my AP prep book.

There are two solubility equations:

CaCO3 (s) <--> Ca++ (aq) + CO3-- (aq) Ksp = 2.8 E -9

CaSO4 (s) <--> Ca++ (aq) + SO4-- (aq) Ksp = 9.1 E -6

part a is fairly straightforward, it asks what the concentration of CO3-- is in a saturated 1.00 L solution of CaCO3.

Part b seems easy but I'm unsure. Excess CaSO4 is added to the solution from part a. Part i of part b asks what the concentration of SO4-- is, and part ii of part b asks what the concentration of CO3-- is.

Now the books answer sheet assumes that the concentration of Ca++ and SO4-- are the same, so it takes the sqrt of Ksq for the second reaction as the concentration of both SO4-- and Ca++. How can the book make this assumption? Wouldn't you also get a viable answer if you assumed Ca++ and CO3-- concentrations were the same? Furthermore, couldn't you just pick some random concentration of any of the 3 substances and then calculate all the others from there to get a "valid" equilibrium point? I was thinking you could just take the 4th root of K1*K2, and then all the substances would be at equal concentration at equilibrium. Obviously Carbonate is not in excess, so you can't just assume that the concentration of carbonate is super high, but sulfate and calcium are in excess, so you could theoretically have a bunch of different combinations of concentrations in order to reach equilibrium (I think?). Am I not seeing something or is this problem somehow flawed?

I was in the lab and was doing an Experiment that involved converting one isomerer into the other. (Maleic acid into fumaic acid). I mounted a reflux system. The idea behind it was simple, simply dissolve maleic acid in water and then add HCl, after that heat it up for about 30 minutes, after it cools of there will be precipitation of fumaric acid. But this did not happen, in fact we got no precipitation. We repeated the experiment with the exact same conditions and this time it was a sucess.

Does anybody have an ideia why it failed the first time? It might have been that we did not heat it for long enough or maybe the vapor whit maleic acid was escaping somehow. But i really dont know.

Hello, I'm trying to figure out if some data collected in our lab is of good quality. While I'm searching for data in literature, I wanted to figure out if the measured pressure is ok relative to an ideal gas.

My test is run in a 100cc cell. I have 25ml of water in the cell at 45C (vapor pressure of H2O at that temperature is 1.5psia).

I inject a known amount of CO2 into the cell (let's say 2g). The pressure of the cell is then measured after equilibrium is reached. The intention of this experiment is to progressively increase CO2 to estimate solubility (and Henry's coefficient of the mixture).

To calculate the ideal gas pressure, I did the following:

Available volume for gas: 100 - 25 = 75cc.

Assuming gas phase is mostly comprised of CO2 since the temperature is above the critical temperature of CO2:

P = (2/44.01) x 8.314 x 318.5/(75*1e-6)

The result is 232psia. Since CO2 dissolves in water, the actual vapor should contain less than 2g CO2 and should always have lower pressure than what I calculate.

Is this calculation appropriate for this situation?

Hello,

In my textbook, I saw that the solubility of salts like AgCl can be increased via complexing with ammonia. I saw they used assumptions that all the Ag+ would be complexed (due to much higher comparative concentrations and the very high K for complexing)

Then, to find the overall K for the reaction, they just multiplied the two constants together 'step wise.' I seem to be finding online that the stepwise constants are usually applied to metal complexes. Does this also occur with any other equilibrium constant? For example when Mg(OH)2 dissociates, is it a stepwise constant multiplication between Mg(OH)2 -> OH + Mg(OH) and Mg(OH) -> OH + Mg2+

Thanks!

Note: I take Pure Chem + Bio. All Physics tips are courtesy of my classmates and this community (https://www.reddit.com/r/SGExams/comments/qqr4pn/o_levels_pure_physicscombined_science_p1_tips/)

3rd last exam day till the end. Might be your last day for some. Either way, hope you had a good rest yesterday. Pure Physics starts at 8am, so make sure to head to school earlier.

Papers today:

- 6091/01 Physics P1 08:00 - 09:00

- 5076/01 Science (Phy/Chem) P1 14:00 - 15:00

- 5077/01 Science (Phy/Bio) P1 14:00 - 15:00

- 5078/01 Science (Chem/Bio) P1 14:00 - 15:00

Things to bring:

- entry proof

- NRIC

- thermometer

- TraceTogether token

- stationery (including pencil + eraser)

- calculator

Feel free to give any tips in comments.

Meta:

- Follow your teacher's advice

- Check subject code, especially for Combi Sci students

- Remember to shade your answers

- Read every question clearly and note keywords

- Pace yourself, aim for 1.5 min/mark

- Allocate time to check

- Note CLT, especially for Combi Chem

- Watch your time. Glance at a stopwatch or the classroom clock after every question if you're time-blind

- Take deep breaths when nervous, before starting the paper and before checking answers

- Stay confident, avoid second-guessing your answers if you're sure earlier

- Don't sleep during the paper itself

- Check you shaded what you intended to shade, write the options on the insert earlier

- It's possible to have multiple options consecutively, 5 'C' in a row for example, don't change your answer just because there's too many of the same options in the row (check base on qn itself)

- Use elimination method for MCQ

- First answer that looks correct isn't always the correct answer

- Skip first and come back later if you don't know the answer to a question

- Don't spend 10-15min on a question, if you might need more than 5min, better to skip first

- If you still don't know, pick a random answer, never leave any question blank

- For Combi Sci, don't neglect either booklet or subject

- Don't do mental calculations in your head, write down your workings if needed

- Using information = answer must be directly derived from qn

Phy (tips all from others, tailored for P1 Phy, some may be applicable to P1 Combi)

- Anticipate a

I'm a high school chemistry teacher (Australia). In an effort to better teach my students next year I'm trying to gain a more comprehensive understanding of the nature of solutions (ionic solutions in particular but all solutions more generally). Much of our senior course focuses on solution chemistry and various equilibrium systems, thus the topic of solubility product ( Ksp) is explored.

What I am trying to nail down is exactly why a solution becomes saturated. My naive notion is that this is a thermodynamic process (and is probably explained by Gibbs free energy). I understand that solubility and dissociation are separate concepts but in the case of ionic substances there are similarities.

The degree to which a solute dissolves in a given solvent is determined by myriad factors , but most notably in this case is the dielectric constant of the solvent, the coulombic attraction between ions and of course the temperature. This does well to describe the magnitude of energy involved in the steps of dissolving, and why some substances would dissolve in different amounts, but not why the process halts at different values for different substances.

My understanding is that the solute will dissolve if it is energetically favourable to do so. By that I mean the Gibbs free energy is negative. Is the saturation/ equilibrium point of any given (ionic) substance, whether highly soluble or sparingly soluble, one of these drivers of the reaction turning the Gibbs positive? Is work done? What exactly is causing the process to stop?

Please correct any of my misunderstandings and point me in the direction of good information to further my own education.

I don't want to step on anybody's toes here, but the amount of non-dad jokes here in this subreddit really annoys me. First of all, dad jokes CAN be NSFW, it clearly says so in the sub rules. Secondly, it doesn't automatically make it a dad joke if it's from a conversation between you and your child. Most importantly, the jokes that your CHILDREN tell YOU are not dad jokes. The point of a dad joke is that it's so cheesy only a dad who's trying to be funny would make such a joke. That's it. They are stupid plays on words, lame puns and so on. There has to be a clever pun or wordplay for it to be considered a dad joke.

Again, to all the fellow dads, I apologise if I'm sounding too harsh. But I just needed to get it off my chest.

I know it's common practice to include some form of air circulation inside of a kegerator to avoid temperature gradients and yet I've avoided doing it for the 5+ years since I've built it. Curious of the effect it would have I used my Bluetooth grilling thermometer with a wired temperature probe to see the effect.

Equipment:

I have a pretty standard kegerator build: 7 cu ft chest freezer, 5.5 in tall collar, 4 taps. Fridge temperature is controlled using an inkbird set to 38F. I placed the temperature probe (iGrill mini) on top of one keg pointing slightly upward into the collar space above them. I bought a variable speed computer fan (link below) so I wouldn't be stuck using a fan running at it's full speed, contributing to extra noise.

Setup:

I placed the fan in various locations within the kegerator and noted the temperature when the system reached equilibrium. The linked image shows the raw data over time.

Results:

Temperature Graph

1 - Control. Fan placed at the top of the kegerator on top of a keg and turned off. Temperature: 53F. 2 - Same as a #1 but fan turned on, highest speed. Temperature: 48F. This was quite loud, I could hear the fan noise easily from 20 feet away. 3 - Fan moved to sit on the compressor hump, lowest speed. Temperature: 43F. Noise was considerably lower. 4 - Fan moved to the bottom of the kegerator, lowest speed. Temperature: 38F. Noise undetectable at 8 feet away.

Results:

Other than the fan creating an even temperature throughout the fridge that matched my inkbird setpoint, I've noticed my pours being considerably smoother on the first pull. Usually the first pour after the kegerator has sat idle for ~1 hour had a little sputtering and slight foaming at the start before pouring cleanly. Now the first pour behaves exactly the same as the 2nd, 3rd, 10th time with smooth flow and zero additional foaming. This is likely due to the dissolved CO2 remaining in solution instead of becoming less soluble over time as the beer in the lines closer to the top of the kegerator gradually increased in temperature to 53F.

Conclusion:

I'm dumb for not doing this sooner.

Note: Not affiliated with any of the products used here, buy whatever you like that fits your needs and budget.

Fan used: https://www.amazon.com/dp/B08TWXW5H8?ref=ppx_pop_mob_ap_share

Do your worst!

I'm surprised it hasn't decade.

I got some requests to post about what equations I wrote and rewrote while studying for this exam.

I am in no way saying you need to memorize all these formulas. However, I really wanted to feel comfortable and was shooting for a top score, so I did. Figure out what works best for you, though!

Also, I was enrolled in physics II as I was studying for the MCAT. So, the physics equations didn't require too much memorization on my behalf.

P.S. My brain is no longer in MCAT mode, so if any equation is wrong or anything looks odd feel free to let me know. I want this to be a great reference document for others :)

Biochemistry:

1. Michaelis Menten equation: V = Vmax[S]/Km+[S]
2. Vmax =Kcat[E]
3. Catalytic efficiency = Kcat/Km *greater this value, more efficient the enzyme
4. Hill coefficient > 1, positive cooperative binding

Hill coefficient < 1, negative cooperative binding

Hill coefficient = 1, no cooperative binding

5. V = E x z/f *migrational velocity = electric field x net charge/frictional coefficient

6. of stereoisomers = 2^n *n is the number of chiral centers

7. Pi = iMRT *osmotic pressure = van't hoff factor x molarity x ideal gas constant (8.3 or .08 depending) x temperature (in Kelvin)

8. Did I memorize the Nernst or Goldmann-Hodgkin-Katz equations? hahaha no. If I was given the equation on test day and asked to solve for it given the other variables, would I be able to? Yes! Nernst equation is used to determine the membrane potential for an ion, while Goldmann is used for determining the overall membrane potential taking into account all ions.

Constants:

Avg molar mass of an amino acid = 110 daltons

General Chemistry:

1. E = hf *Energy of a photon = planck's constant x frequency
2. E = -R/n^2 *Energy of an electron = rydberg constant/principal quantum number^2
3. E = hc/λ *Energy of a photon = planck's constant x speed of light/wavelength
4. p = qd *Dipole moment = charge x distance separating charges
5. percent yield = actual yield/theoretical yield x 100%
6. rate = z x f *Rate of rxn according to collision theory = total number of collisions/second x fraction of effective collisions
7. k = Ae^(-Ea/RT) *Rate constant (Arrhenius equation) = frequency factor x e^(- activation energy x ideal gas constant x temperature in Kelvin)
8. [A]t= [A]0e^(-kt) *Concentration of radioactive isotope at some time = initial concentration x e^(- rate constant x time)
9. Keq = [produ

Previous Episodes:

1. Snowball
2. Opal
3. Rust
4. Vitrium
5. Blue Crystal
6. Cannonball

Still is a cool super-Earth with a mixture of water and ammonia oceans. The surface pressure is approximately 5 bars, with 4.5 bars of nitrogen, 0.5 bars of hydrogen, and trace quantities of methane, ammonia, and water vapor. The average global temperature is approximately -30C--just above the boiling point of ammonia at 1 atmosphere--but varies between daytime highs of just over 0C at the equator and -50C at the poles.

Life in mixed water and ammonia solution faces numerous chemical challenges--it must select from the set of potential macromolecules which are stable against reaction with both media. Nevertheless, the cosmic abundance of both water and ammonia, and their effects on increasing each other's liquid ranges, means such systems are common among hydrogen-rich terrestrial worlds.

The functional equivalents of sugars, and other reactive biomolecules, in Still biology are all strongly, but not entirely, nitrogenated, with amine an amidogen groups replacing =O and hydroxide groups. The basic glucose analog has the formula C6H15ON5, and forms nitrogenous starches through linking primary amide groups to form secondary amines and release ammonia. Hydrogen-exchanging photosynthesis and respiration proceed according to the equilibrium

6 CH4 + 5 NH3 + H2O <=> C6H15ON5 + 13 H2

The detailed sub-reactions are about as unexpected as the are for oxygenic photosynthesis on Earth. Despite the suggestive elemental ratios, photosynthesis on Earth does not proceed by splitting oxygen from CO2 and combining it with water, but by splitting water and using the hydrogen to attack CO2. Similarly, photosynthesis on still proceeds by splitting hydrogen off of ammonia (a considerably less energy-intensive process than splitting water) and using reactive amine and imidogen groups to attack water and methane. The genetic molecule is also highly analogous, using nitrogenous sugar backbones with phosphate linkages providing the repeat

For context I'm a Refuse Driver (Garbage man) & today I was on food waste. After I'd tipped I was checking the wagon for any defects when I spotted a lone pea balanced on the lifts.

I said "hey look, an escaPEA"

No one near me but it didn't half make me laugh for a good hour or so!

Edit: I can't believe how much this has blown up. Thank you everyone I've had a blast reading through the replies 😂

It really does, I swear!

Might be a super stupid question...

I thought Km was the substrate concentration at which 50% of the enzymes were bound. Half of Vmax, and then go over to x to find it. Lower Km=better affinity

But then Kd appears to be the same thing? I thought it was the disassociation constant, so lower the Kd higher affinity, but not sure what it actually means

They’re on standbi

Pilot on me!!

Nothing, he was gladiator.

Dad jokes are supposed to be jokes you can tell a kid and they will understand it and find it funny.

This sub is mostly just NSFW puns now.

If it needs a NSFW tag it's not a dad joke. There should just be a NSFW puns subreddit for that.

Edit* I'm not replying any longer and turning off notifications but to all those that say "no one cares", there sure are a lot of you arguing about it. Maybe I'm wrong but you people don't need to be rude about it. If you really don't care, don't comment.

When I got home, they were still there.