# Finding volume content

## Homework Statement

The question is as following:

5cm3 of oleic acid are mixed with 95 cm3 of methanol. 5 cm3 of this mixture are
then diluted with a further 95cm3 of methanol, achieving an overall dilution of oleic
acid of a factor 400. A method is perfected of creating identical droplets; counting
out 97 such droplets gives a total volume of 1 cm3. What is the volume of oleic acid
contained in one droplet of the mixture?

## The Attempt at a Solution

I have absolutely no idea on how to solve this problem. Usually problems indicate some quantity which we can substitute into an equation.
Any tips will be greatly appreciated.

cepheid
Staff Emeritus
Gold Member
You know that the amount of oleic acid in the final mixture is 1 part in 400. So all you have to do is figure out the volume of a single droplet, and the volume of oleic acid in that droplet will be 1/400 of that droplet volume.

So, what is the volume of a droplet? Well, 97 droplets have a volume of 1 cm3, so 1 droplet has a volume of ____?

You know that the amount of oleic acid in the final mixture is 1 part in 400. So all you have to do is figure out the volume of a single droplet, and the volume of oleic acid in that droplet will be 1/400 of that droplet volume.

So, what is the volume of a droplet? Well, 97 droplets have a volume of 1 cm3, so 1 droplet has a volume of ____?

Hm will it be 1/97 cm$^{3}$ , if so then would I multiply this fraction by 400 ?

cepheid
Staff Emeritus
Gold Member
Hm will it be 1/97 cm$^{3}$ , if so then would I multiply this fraction by 400 ?

That's right.

That's right.

Thank you very much! The wording of the question brought about confusion to my mind.This question has another parts as well, which I'll try to solve.

SteamKing
Staff Emeritus
Homework Helper
Are you sure multiplication by 400 equals the correct volume of oleic acid?

Are you sure multiplication by 400 equals the correct volume of oleic acid?

Volume of oleic acid in one droplet ?
why is it wrong ?

SteamKing
Staff Emeritus
Homework Helper
Because the oleic acid was being diluted in methanol.

cepheid
Staff Emeritus
Gold Member
Are you sure multiplication by 400 equals the correct volume of oleic acid?

Yeah, sorry, I missed that. I thought he meant multiply the denominator by 400 (i.e. multiply by 1/400 just the way I told him to).

Because the oleic acid was being diluted in methanol.

Yeah, sorry, I missed that. I thought he meant multiply the denominator by 400 (i.e. multiply by 1/400 just the way I told him to).

Oh I see.. I initially thought of multiplying by 1/400 , i even had that posted as an answer until I changed my mind.Some misunderstanding from both ends. Now it's clear. Thanks.

So here's my final answer for this question: 1/97 x 1/400 ~ 2.6x10^-5 cm3

The questions which follow on are:

The surface of a tank of water is dusted with fine powder. A single drop of the
diluted mixture is allowed to fall on the water surface. The alcohol component mixes
with the water and the oil forms a monolayer film on the water surface, displacing
the dust so it is clearly visible, with a diameter of 9.4 cm. What is the thickness of
the film?

I have answered this: All that was needed to be done was using the Volume = Area x thickness.My final answer is: 3.7x10^-7 cm.

Next question's:
The shape of an oleic acid molecule can be approximated by a cuboid l ´ l ´ 7l .
One end (COOH) loves water (hydrophilic) and the other end hates it (hydrophobic).
Assume that these molecules stick out of the water and form a monolayer close
packed in a square arrangement. What are the dimensions of the molecule?

What does the statement which I have highlighted in bold mean ? do I find out the unit dimension of the molecule? I know lxlx7l denotes volume of the molecule.

Last edited:
Never mind I have found the answer for this. Thanks everyone for your help!

I need a quick tip on how to solve one of the last bit of this question. The question basically wants me to derive Avogadro's number of oleic acid. So the molecular mass I have got is : 282.
Now I know moles = Mass/ Mr.
To get mass I will have to use density = Mass/ volume.
But once I sub in the numbers.. I get something in the range of 5x10^24! :(

My volume for oleic acid in one droplet was: 2.6x10^-5 (cm3 , do i divide this by 100 to get m3, or is it 1x10^-6 ? )
Density in the question is given to be 890 kg/m3.

Any help will be much appreciated. Thanks!

How do I find out the volume of something that is 36w - 16h - 16d. Would it just be 36 * 16 * 16.