DAT #181 Chemistry

[woox]

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Just finished Destroyer and have a few problems I am completly stuck on that I will be asking on the forums.

I don't understand the attached problem (and solution), to #181.

My solution:

I was able to setup a simple math equation to solve this one...but I doubt I would have figured it out unless I saw the answer first so I would like to know proper way to do this.

My work:

(0.2)/(0.03) = (X)/(0.025)

(cross multiply and solve for X)

X= 0.167

Questions attached below:

 

[spoog74]

Just finished Destroyer and have a few problems I am completly stuck on that I will be asking on the forums.

I don't understand the attached problem (and solution), to #181.

My solution:

I was able to setup a simple math equation to solve this one...but I doubt I would have figured it out unless I saw the answer first so I would like to know proper way to do this.

My work:


Questions attached below:

you forgot to consider the 2 ions that Ca(OH)2 dissociates to, meaning you multiply the initial concen by 2 and then solve for x.

 

[woox]

Doing what you said with what I did would not get the right answer.

That is what the solution does, but I don't understand this N/M and the dividing by 3?

 

[rockclock]

Doing what you said with what I did would not get the right answer.

That is what the solution does, but I don't understand this N/M and the dividing by 3?

M is molarity, which is the concentration of the original acid/base
N is normality, which you can think of as the concentration of H+/OH- produced by the original acid/base

in this example,
each barium hyroxide molecule disociates into two OH- ions. if you had 1 M Ba(OH)2 you'd get 2 M OH-...follow?
also citric acid is a triprotic acid, meaning 1 M citric acid would give you 3 M H+

the reason you have to count everything in terms of H+/OH- and not the original acid/base is that one H+ neutralizes one OH-, not necessarily one molecule of acid corresponds to one molecule of base (see above)

so

2 * [Ba(OH)2] * V_Ba(OH)2 = 3 * [citric acid] * V_citric acid

[Ba(OH)2] = 0.2 M
V_Ba(OH)2 = 30 mL

[citric acid] = ?
V_citric acid = 25 mL

 

[ijk90825]

2 hydroxide ions in Ba(OH)2 and 3 H+ in citric acid.
since you need 1 hydroxide for 1 H+,
2(M1)(V1) = 3(M2)(V2)
2(.20M)(30 mL) = 3(M2)(25 mL)

M2 = .16 M but they got .167 cuz they just put .48 as .5

Hydroxide and H+ not in 1:1 ratio so gotta be multiplied as well

 

[woox]

Wow thanks for that explanation...but that is extremely complex - I am assuming something like this wont be on the DAT?

This is the first time I have seen a problem like this (or maybe second), but I don't think Chad vids covered this.

 

[rockclock]

Also, forget your original set-up...you lucked out

 

[rockclock]

Wow thanks for that explanation...but that is extremely complex - I am assuming something like this wont be on the DAT?

This is the first time I have seen a problem like this (or maybe second), but I don't think Chad vids covered this.

no, this is definitely DAT level material. it could easily be on your test.

it's no more complicated than monoprotic acid/monoprotic base neutralization once you realize you have to take into account how many H+ the acid produces and how many OH- the base produces.

50 mL of 1 M HCl neutralizes 20 mL of NaOH...give me the molarity of NaOH...easy right?

this is just one step more complicated than that.

 

[woox]

AAAAAH I remeber now, thanks for that 😀

 

[jwnichols21]

2 hydroxide ions in Ba(OH)2 and 3 H+ in citric acid.
since you need 1 hydroxide for 1 H+,
2(M1)(V1) = 3(M2)(V2)
2(.20M)(30 mL) = 3(M2)(25 mL)

M2 = .16 M but they got .167 cuz they just put .48 as .5

Hydroxide and H+ not in 1:1 ratio so gotta be multiplied as well

I like this way of explaining it much better than Destroyer's explanation..thank you!