MCQ unit 5

Stuck on these below;

I got the correct answer but was wondering why C is not correct either;
ImageUploadedByStudent Room1401126866.222345.jpg

What's the difference between C and D below?:
ImageUploadedByStudent Room1401126965.994738.jpg

Is the rate of formation exponential? Is it due to exponential decay nature of atoms so element X continuously decays into stable element Y, and this rate slows down exponentially...but why can't graph touch the y axis? Does the above still apply when nothing has even decayed?!

Thanks! -Jay

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Reply 1

uberteknik

Original post by Jaydude

Stuck on these below;

I got the correct answer but was wondering why C is not correct either;
ImageUploadedByStudent Room1401126866.222345.jpg

What's the difference between C and D below?:
ImageUploadedByStudent Room1401126965.994738.jpg

Pressure = force per unit area. The first question tells you the boxes are different sizes so the pressure cannot be compared.

Reply 2

Jaydude

Original post by uberteknik

Pressure = force per unit area. The first question tells you the boxes are different sizes so the pressure cannot be compared.

Oh I now understand, what if they were the same though? (As I thought) does both temp and kinetic energy differ? Ie both B and C are correct?

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Reply 3

uberteknik

Original post by Jaydude

Oh I now understand, what if they were the same though? (As I thought) does both temp and kinetic energy differ? Ie both B and C are correct?

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If they were the same surface area then the pressure would be equal.

Reply 4

Jaydude

Original post by uberteknik

If they were the same surface area then the pressure would be equal.

But there's greater root mean squared speed of molecules in P than in Q so shouldn't there be greater pressures exerted on walls? Are they the same because as forces exerted on wall increases the volume does too? Hence the same pressures? I'm sorry for asking so much!

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Reply 5

Jaydude

Bump! Anyone?

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Reply 6

Stonebridge

If you had two identical boxes (same volume) with the same mass of nitrogen in both, and both at the same temperature, then they would both have the same pressure.
pV = nRT
The molecules inside both would have the same mean (square) speed.
That's basic Gas Laws.

If you increase the temperature in one of the boxes, you will increase the pressure.

(edited 9 years ago)

Reply 7

Jaydude

Original post by Stonebridge

Oh, I have to take into account that equation too, PV=NrT. Thank you!

If you don't mind, what about the second question below(and shown above)why is the answer D and not C? I'll post the question again below:
ImageUploadedByStudent Room1401289434.201907.jpg

ImageUploadedByStudent Room1401289434.201907.jpg

Why does the line just have to begin from nowhere in D, how is that correct? Surely it must touch the y axis has at t=0 the rate of formation starts...
Confused :frown:

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Reply 8

Stonebridge

Original post by Jaydude

Why does the line just have to begin from nowhere in D, how is that correct? Surely it must touch the y axis has at t=0 the rate of formation starts...
Confused :frown:

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Can you post a link to where it says D is the correct answer?
At time t=0 the rate of formation of Y will be equal to the rate of decay of X.
At time t=0 the rate of decay of X will have a definite value A = λN where N is the amount (number of active nuclei) of X present.
So I don't see how D can be the correct answer.

We've had cases before where students have been looking at the wrong mark scheme or question number; or just that the mark scheme was incorrect.

Reply 9

Jaydude

Original post by Stonebridge

So sorry, just re looked at the markscheme and C is the correct answer... All that stress for nothing. Shall I post the link as proof if you want? Also thanks again for previous answers!

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