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Experiments show that if the chemical reaction$ N_2 O_5 \to 2NO_2 + \frac {1}{2}O_2 $takes place at $ 45^oC, $ the rate of reaction of dinitrogen pent-oxide is proportional to its concentration as follows:$ -\frac {d [N_2 O_5]}{d} = 0.0005[N_2 O_5] $(See Example 3.7.4.)(a) Find an expression for the concentration $ [N_2 O_5] $ after $ t $ seconds if the initial concentration is C.(b) How long will the reaction take to reduce the concentration of $ (N_2 O_5), $ to $ 90\% $ of its original value?

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02:35

Wen Zheng

Calculus 1 / AB

Chapter 3

Differentiation Rules

Section 8

Exponential Growth and Decay

Derivatives

Differentiation

Oregon State University

University of Michigan - Ann Arbor

Idaho State University

Boston College

Lectures

04:40

In mathematics, a derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much one quantity is changing in response to changes in some other quantity; for example, the derivative of the position of a moving object with respect to time is the object's velocity. The concept of a derivative developed as a way to measure the steepness of a curve; the concept was ultimately generalized and now "derivative" is often used to refer to the relationship between two variables, independent and dependent, and to various related notions, such as the differential.

44:57

In mathematics, a differentiation rule is a rule for computing the derivative of a function in one variable. Many differentiation rules can be expressed as a product rule.

04:57

Experiments show that if t…

02:45

04:27

01:28

06:25

07:34

Dinitrogen pentoxide decom…

01:53

For the first-order decomp…

01:16

The rate constant for the …

03:26

At $45^{\circ} \mathrm{C},…

So we start out with this pretty intimidating looking equation. It's a differential equation because it has a derivative involved in it, and what we need to do is simplify things for ourselves. So let's say that why is equal to the concentration And then let's substitute that into our equation. And what we have is the opposite of de y t t equals 0.5 One more zero in there times. Why now? Hopefully we remember from the reading in the textbook that we were told that the solution to the equation de y d T equals K T, which is really very similar to what we have here, is why of tea equals Why not you to the Katie? In other words, the solution to this differential equation is this exponential growth model. And so what we need to do with our differential equation is just divide both sides by negative one so that we can get it to look more like this. Groups more like this. Okay, so divide both sides by negative one, and we have d y d t equals negative 0.0 y five way and the number negative. 0.5 were calling that k. So then our model is going to be based on this right here is going to be Why equals the initial amount times e to that value of K times time. Now we were told that the initial amount, the initial concentration is capital C. So let's replace our Why not with a capital C and here we have our model. Now we can also remember that we substituted and why is equal to the concentration so we can replace the why with that concentration. And we have what we're looking for for this part of the problem, that is our model. Okay, so we can use that model to solve part B. So for part B, we're told that the concentration is going to be 90% of the original. So 90% of C and 90% of C could be expressed as 900.9 seat. So we're going to put 0.9 c and for the concentration into our equation and sell for the time, how long does it take to reach a 90% of the concentration? So let's divide both sides by sea, and we get 0.9 equals e to the negative 0.5 t. Then we take the natural log of both sides. And then we divide both sides by negative 0.5 So we get time equals natural log a 0.9 divided by negative 0.5 And we put that into a calculator and it is approximately 211. And what were the units of time here? They were seconds, I believe so. It's 211 seconds.

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