## Sapling Question #10

$\lambda=\frac{h}{p}$

Norah Gidanian 3D
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### Sapling Question #10

The average speed of a diatomic fluorine molecule at 25 ∘C is 442.4 m⋅s−1 . What is the average wavelength of a fluorine molecule at this temperature? Assume that the molecule acts as a single particle.

Can someone walk me through doing this? I keep trying to do it and am not getting the correct answer. I think I may be converting wrong or something

Alexandra Ahlschlager 1L
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Joined: Wed Sep 30, 2020 9:59 pm

### Re: Sapling Question #10

The equation I used for this was the De Broglie equation: wavelength = h/mv. We already know the values of h and v (velocity) so the only thing we need to calculate is the mass. Since it’s a diatomic fluorine molecule, we can multiply the molar mass of fluorine by two to get the molar mass of the molecule. This gives us a unit of grams per mole, but we actually need kilograms per molecule. Using the conversion for moles, we can divide the molar mass by avogadro’s number to get the mass of an individual fluorine molecule in grams. The last thing we need to do is convert from grams to kilograms by dividing by 10^3. From there, you should have all the values you need to solve for wavelength. Hope this helps!

Brianna Martilla 1C
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### Re: Sapling Question #10

Use lambda=h/mv. h is a constant and velocity is given to us. To find m, divide the mass of two fluorine molecules by Avogadro's number to get the mass of one particle, then convert that value to kg because h, Planck's constant, is in units of Js, or kg*(m^2)*(s^-2)*s. Lastly, remember to use the correct number of significant figures. Hope this helps.

Jiapeng Han 1C
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### Re: Sapling Question #10

Firstly, you need to calculate the mass of a single fluorine molecule. To do that, you could divide the molecular mass(this is in g) of fluorine molecule by the Avogadro number. Please note that you need to convert g to kg! Once you have the mass of a single fluorine molecule, you could use the formula wavelength=planck constant/(mass*velocity) to get the value of wavelength.

DominicMalilay 1F
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### Re: Sapling Question #10

As a good baseline, whenever I run into problems involving the velocity/speed of something, I think of the De Broglie equation!

RitaThomas_3G
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### Re: Sapling Question #10

Hey! So these are the steps that I went through to do this question:

Step 1: Decide which equation to use. Since I am solving for wavelength, and seeing the given values in the problem, I decided to use De Broglie's Equation: $\lambda = \frac{h}{mv}$.
Step 2: Write down given values. First, we know that h is a constant. Second, we can also see that v was given for us in the problem.
Step 3: Solve for other unknown values. After writing down all the immediate given values, we can still see that in order to solve for wavelength, we still need mass. However, we can see that we can solve for it since the name of the molecule is diatomic Fluorine. So, I will convert this into its molar mass and divide it by Avogadro's number. Also, I will convert it to kg since that is the units in Planck's constant. So, I will divide it by 10^3.
Step 4: Solve! Plug in all the values you have now and solve for wavelength. Be careful about plugging in the right numbers and using correct sig figs.

nayha a 1E
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### Re: Sapling Question #10

Sabrina Galvan 3J
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### Re: Sapling Question #10

nayha a 1L wrote:Everyone in this thread is mentioning Avogadro's number, but as I look through my notes, I can't seem to find anything about it. Does anyone know which lecture talked about this?

It most likely is from one of the first lectures we had on fundamentals, but the textbook explains Avogadro's number on section E of the Fundamentals section!

Chance Herbert 3A
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Joined: Wed Sep 30, 2020 9:55 pm

### Re: Sapling Question #10

nayha a 1L wrote:Everyone in this thread is mentioning Avogadro's number, but as I look through my notes, I can't seem to find anything about it. Does anyone know which lecture talked about this?

Avogadro's Constant is referenced near the beginning of the October 2nd Lecture! (Listed in Week 1 I believe)