TBR chem kinetics

[lamborghiniMD]

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What's an easy way to solve this question, mathematically:

1. Given the following data, what is the concentration of X at t = 3?

t: [M]:
0 1
1 .56
2 .35
4 .25


The answer is .28M, but I just want to know an easier way to solve this question mathematically. Thanks!

 

[MissionStanford]

It seems like there should be more information. My logic would simply be to pick something between .25 and .35 since the trend is a decrease in [M] as time passes, and .35 is given for t = 2 while .25 is given for t = 4, so t = 3 should be between the values. If there's more information, I may be able to help out more.

Also, I should have mentioned that the decrease in [M] over time happens at a slower rate each time. From t = 0 to t = 1 you got from 1 to .56 (down by .44) while from t = 1 to t = 2 you go from .56 to .35 (only down by .21 this time). Thus, you can also logically conclude that at t = 3 that the value will be closer to .25 than to .35

 

[LoLCareerGoals]

What's an easy way to solve this question, mathematically:

1. Given the following data, what is the concentration of X at t = 3?

t: [M]:
0 1
1 .56
2 .35
4 .25


The answer is .28M, but I just want to know an easier way to solve this question mathematically. Thanks!

What is the order of kinetics here? It is def not first order, I just did a plot of ln[M] vs time and it is not linear. You can do a curve fit to determine kinetics order, then using that model, compute the point #3, but doing so on paper at test time would be insane.
Are you sure there is no more info?

 

[sciencebooks]

It seems like there should be more information. My logic would simply be to pick something between .25 and .35 since the trend is a decrease in [M] as time passes, and .35 is given for t = 2 while .25 is given for t = 4, so t = 3 should be between the values. If there's more information, I may be able to help out more.

Also, I should have mentioned that the decrease in [M] over time happens at a slower rate each time. From t = 0 to t = 1 you got from 1 to .56 (down by .44) while from t = 1 to t = 2 you go from .56 to .35 (only down by .21 this time). Thus, you can also logically conclude that at t = 3 that the value will be closer to .25 than to .35

👍 I agree. When there's sufficient information, math is best. But honestly, this is sometimes the best you can do. I try not to overthink it.

 

[lamborghiniMD]

This was all the TBR gave us. This was a discrete question and the that's literally all they wrote. They said it was 2nd order kinetics, and made up some reasoning, but I feel like on the MCAT I wanna make accurate estimations rather than "guesstimations" maybe I shouldnt worry about this problem then? lol

 

[ebasappa]

I remember this question, from the info you can see it was a second order question. In second order kinetics the RATE decreases, so between the two seconds there is a .10 drop. Because the rate drops, the drop from t=3 to t=4 should be lower than the drop from t=2 to t=3, and .28 fufills that.

BTW, the way u know it's second order is that had it been a first order, the decrease between each interval is exactly HALF the last, in this data, each drop is decrease not uniform (ie: a .44 drop, then a .21 drop, then a .1 drop between TWO intervals)