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### Topic: How to calculate the cross-sectional area of stearic acid  (Read 3434 times)

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#### FancySnow

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##### How to calculate the cross-sectional area of stearic acid
« on: February 15, 2016, 09:06:05 PM »
Hi chemistry experts, I need your help.

One of my pre-lab question ask me to calculate the cross-sectional area of stearic acid.

The lab manual states:
Quote
In order to do this, you will first need to develop a method for estimating the cross-sectional area and the length of a stearic acid molecule. You can estimate both using a bit of geometry and by approximating the stearic acid molecule as a chain containing 18 carbon atoms with a bond length of 154 pm and a bond angle of 109.5o. The carbon atoms are in a zig-zag pattern as shown in the figure above.

I'm confused because we have Monday off for holiday so the professor never went over it but the lab is tomorrow. Can anyone shed light on this step by step?

Thanks!

#### Borek

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##### Re: How to calculate the cross-sectional area of stearic acid
« Reply #1 on: February 16, 2016, 02:58:43 AM »
This is about a simple geometry. Try to draw a molecule and see if what you know about triangles, angles, line segments, distances between points on a plane, can be applied to finding the molecule length.
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#### FancySnow

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##### Re: How to calculate the cross-sectional area of stearic acid
« Reply #2 on: February 16, 2016, 04:43:12 AM »
This is about a simple geometry. Try to draw a molecule and see if what you know about triangles, angles, line segments, distances between points on a plane, can be applied to finding the molecule length.

I checked online and a triangle with 109 degree is the isosceles triangle with an area formula of A = (bh)/2

How do I know if 154 is the base or height? How can I find the other length?

#### mikasaur

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##### Re: How to calculate the cross-sectional area of stearic acid
« Reply #3 on: February 16, 2016, 01:04:49 PM »
Soh-Cah-Toa!

I'm pretty sure the right angle trigonometry section at the top is all you'll need. As a hint, you know $c$ directly and can find angle $u$ pretty easily.
Or you could, you know, Google it.

#### Enthalpy

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##### Re: How to calculate the cross-sectional area of stearic acid
« Reply #4 on: February 16, 2016, 03:24:42 PM »
The cross-sectional area for what kind of interaction?

Electrons of some energy? X rays of some energy? Alphas? Visible light? Collisions of the molecule with itself? The values are completely different.