A little help please

[ingramw1202]

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Okay I have a few questions over things I'm having trouble with.

1. What is the difference between the radioactive particles? Like alpha beta and gamma.

2. How can you relate concentration of acids and bases to the pka? If you have a concentration of like 0.1M NaOH in solution, how do you determine the pka?

3. What is the optimal ph range for an indicator? Like what determines the best indicator to use?

4. How do you determine which is the oxidizing agent and reduction agent in a reaction like this: 2hcl + MnO2 + H2O2 ----> Mncl2 + 2h2o + o2

5. If you have an object initially at rest and then you apply a force over a distance to give the object an initial velocity and it goes vertically only to reach a peak height of h and then explodes Into pieces, what is the total momentum of all the pieces? I couldn't figure this one out. Is it still zero even though the object has work done on it?

If anyone knows any shortcuts for these id greatly appreciate it. My problem now is with time on the ps section.

 

[Trauma16]

1. With radioactive particles, all you have to do is keep track of atomic numbers of molecular masses, which is much easier if you just think of it as a simple chemical reaction. In alpha decay, an atom loses a helium atom (mass of 4, number of 2) so the products will be the alpha particle (a helium atom), and the initial reactant, but now with an atomic number smaller by two, and a mass smaller by 4. Beta decay is simply the explusion of an electron, so the beta particle is an electron. Positron emission is another type of beta decay, but in this case the particle is a positron, which the antiparticle of the electron (it has a positive charge rather than a negative charge). Gamma rays are the easiest, since the gamma particle is a very high energy photon, which has no mass and no charge. Thus, an atom that undergoes gamma decay is unchanged.
To summarize alpha particle = helium atom, beta particle = electron, gamma particle = uncharged, massless photon (light). Also, remember that a lot of radioactive questions ask you to determine momentum after the decay, so just remember the types of particles and their masses and you'll be fine.

2. Think of Ka like the eqiulibirum constant except for acids and bases. The actual equilibrium reaction is HA --> H + A, which is the loss of the acids proton to form a proton and the acids conjugate base. Thus, like the equilibrium equation, you just multiply the concentrations of the products, and divide it by the multiplied concentration of the reactants, so Ka = [H][A]/[HA], and like pH, pKa = -log[Ka]. The pKa of solutions is not something you can find; they are values that are determined experimentally, and, as a researcher, you would use tables of these values to solve questions. Thus, the pKa of NaOH is a defined value; a question you might receive is, "At equilibrium, a 0.1 M solution of NaOH has 0.04 M of OH ion, 0.04 M of Na ion, and 0.02 M of NaOH. Calculate the pKa."
So, with a given pKa, concentration of protons is directly related to pKa: lower pKa implies more protons.

3. The optimal range of an indicator is one that has its endpoint (the concentration at which it changes color) in the same range as the equivalence point of the titration (the point where there are equal concentrations of acid and base).

4. Oxidation is loss of electrons, reduction is gain of electrons; OIL RIG. The atom that is oxidized provides the electrons for the atom that is reduced, and is therefore the reducing agent; conversely, the atom that is reduced takes electrons from the atom that is oxidized, and is therefore the oxidizing agent. In this reaction, Mn is going from +4 to +2, and oxygen is going from -2 to 0. Thus, Mn gains electrons and is reduced, implying it is the oxidizing agent. Oxygen loses electrons, and is oxidized, implying that it is the reducing agent.

5. Since the momentum starts at 0 at the explosion, by conservation of momentum, the momentum of the pieces taken together will still be zero. Technically, work was not done on the object because a force was not applied to it over a distance. Instead, chemical potential energy (in the explosive material) is converted into the kinetic energy of the pieces. Since no external force is done on the box itself, conservation of momentum holds, and the momentum is 0.
For this, just remember if no external force is acting on a system, then conservation of momentum holds.

I hope some of this helped, and good luck with the MCAT!!

 

[member232]

1. What is the difference between the radioactive particles? Like alpha beta and gamma.
alpha: A =4, Z =2 So if you were to have alpha decay, your parent atom would have 4 less in the mass number and 2 less in the proton number
beta: A = 0, Z= -1 Same principle as above, except the mass number would not change and the proton number would add one
gamma: doesn't change anything mathematically; conceptually its the emission of a high energy photon

*note: the reverse processes can occur in which the radioactive particles are absorbed... this kind of stuff is just algebra, really


2. How can you relate concentration of acids and bases to the pka? If you have a concentration of like 0.1M NaOH in solution, how do you determine the pka?

This is where the Henderson-Hasselbalch equation comes in:

pH= pKa + log [A-]/ [HA] OR pOH = pKb + log [BH+]/

So, if you have a 0.1 M NaOH solution, your concentration of OH is 0.1. You need to be given either the pKa, or pH to find the other one. (same applies for bases) Remember, pH + pOH = 14, if you want to switch between the two.

3. What is the optimal ph range for an indicator? Like what determines the best indicator to use?

Each indicator has a distinct pH range in which it will be deprotonated. Indicators are typically weak acids or weak bases themselves. The protonation/deprotonation of the indicator causes it to change color. Here's a scenario to see if things are a bit clearer: Imagine you have 2 M NaOH. You are titrating it with HCl. After you have added 2 moles of HCl, you reach equivalence point. At equivalence point, the pH is 7 because all of the HCl has deprotonated to Cl- and OH- has paired with H to form H2O. Because you know you are reacting a strong acid with a strong base, you should pick an indicator that has an optimal range that includes 7, and is maybe lower than 7. Because you started off with a basic solution, you are going from a high pH, to neutral pH. As soon as you add a drop of HCl more than 2 moles (equivalence point) the acid will protonate the indicator you chose (which is conceptually a weaker base than OH) and thus the color will change.

4. How do you determine which is the oxidizing agent and reduction agent in a reaction like this: 2hcl + MnO2 + H2O2 ----> Mncl2 + 2h2o + o2

Mn is going from +4 to +2, and is getting reduced and thus is the oxidizing agent.
O is going from -1 in H2O2 to 0 in elemental O2, and is getting oxidzed, so H2O2 is the reducing agent.

5. If you have an object initially at rest and then you apply a force over a distance to give the object an initial velocity and it goes vertically only to reach a peak height of h and then explodes Into pieces, what is the total momentum of all the pieces? I couldn't figure this one out. Is it still zero even though the object has work done on it?



Its zero because right before explosion, the object is at the peak of its parabola where v=0. thus, momentum right before explosion is m1v1 and velocity is 0, so there is no momentum. as momentum is conserved, the final momentum will also be zero

Hope that helps!

 

[member232]

didn't see Trauma16s answer! sorry about that, hope you're able to synthesize the information 🙂

 

[ingramw1202]

Wow thank you so much. I really really appreciate your response. I generally do well on ps but I took aamc 10 today and freaked out. Made a freaking 9. Dropped four points lol. But this clears it up. Thanks again!