[joocystacks]

I'm befuddled by this ? 

1. Angles between the carbon bonds are 109 degrees
2. Bond length between two carbons is 1.54 angstrom

calculate the length of the carbon backbone in stearic acid.

how do you set this up?

[Schrödinger]

There are 2 ways to approach this :

1. 'Length of the backbone' can refer to the sum of lengths of each bond, which is quite simple


2. On the other hand, it could represent the horizontal distance between the first and the last carbon, which is not quite simple.

i.e.,'length of backbone' could refer to the length of the red line in the following pic :



In this case, you need to use the properties of isosceles triangles to find the horizontal length between 2 carbon atoms, hence you need to make use of the bond angle