Ph

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What's the best way to solve 1337 and 1339

I always get messed up with PH

I don't get eat their explanations

 

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How come they are multiplying h2co3 by 2. Shouldn't it be the naoh

 

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How come they are multiplying h2co3 by 2. Shouldn't it be the naoh

Or is there another way to do it

 

[deleted647690]

For 1337, A corresponds to the half equivalence point (half of the first equivalence point). The first equivalence point is the point at which the number of mols of base added equals the number of moles of acid titrated.
So if you calculate the number of moles of acid given:

500 ml NaOH (.75 M) = 375 moles

Then you want to calculate how many moles of NaOH would be required to get 375 moles

So you are given 2.25 M NaOH

So 2.25 M NaOH (x ml) = 375 moles

x=166.66 ml

This is the amount needed to reach the first equivalence point, and we want the amount needed to reach A (half of the first equivalence point). So divide by 2, and you end up with 83 mL

 

[Squish miss]

For 1337, A corresponds to the half equivalence point (half of the first equivalence point). The first equivalence point is the point at which the number of mols of base added equals the number of moles of acid titrated.
So if you calculate the number of moles of acid given:

500 ml NaOH (.75 M) = 375 moles

Then you want to calculate how many moles of NaOH would be required to get 375 moles

So you are given 2.25 M NaOH

So 2.25 M NaOH (x ml) = 375 moles

x=166.66 ml

This is the amount needed to reach the first equivalence point, and we want the amount needed to reach A (half of the first equivalence point). So divide by 2, and you end up with 83 mL

thanks. do you get this. it says that greater the area, the least number of fracture. but the graph seems to say something different. it seems to me that the number of breaks increases with more area

 

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continued

 

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@Next Step Tutor @NextStepTutor_1

 

[NextStepTutor_1]

Hi AnteaterPediatrician,

Thanks for the questions!

For #1337, the explanation by @acetylmandarin covers it, but feel free to ask if you have further questions. For #1339, the key is to leverage the info given in the table. The conceptual idea in the background here is that pKw is not always 14.0 -- in fact, although we often use that explanation, it only holds true for standard conditions (~25°C). As the temperature increases, the pKw decreases. This is shown in the table. For the MCAT, you definitely don't need to know specific non-standard values of pKw, but you should know that Kw—like any other equilibrium constant—can vary with temperature, which means that the pH of neutral water also varies with temperature. The idea here is that a neutral substance is defined by having the pH = pOH. Combining this with the equation pH + pOH = pKw, we use the value they give of pKw for 75°C (12.7) and solve, obtaining the equation 2 * pH = 12.70 --> pH = 6.35. The real lesson to be learned here is the temperature dependence of Kw and its implications for which pH levels we consider "acidic" or "basic".

Re: Q8, it sounds like the challenge may have been keeping track of angle to the normal (i.e., a vertical line perpendicular to the ground) and angle to the ground itself. This definitely underscores the importance of paying close attention to passage wording re: these issues, and you will also find that angles defined relative to the normal are common in optics.

Hope this helps!

 

[deleted647690]

thanks. do you get this. it says that greater the area, the least number of fracture. but the graph seems to say something different. it seems to me that the number of breaks increases with more area

I think NS Tutor answered it well above. I honestly probably would have missed this one.....haha

 

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I think NS Tutor answered it well above. I honestly probably would have missed this one.....haha

but in the graph, its sigmoidal. the graph is going to the right. doesn't that imply more breaks (which is bad) with a grater area??? is the breaks per thousand good or bad/

i get the angle., just not the cross sectional area