Posted by
**Hol** on
.

"a drop of oil (volume 0.05cm3) is released from a medicine dropper on to the calm surface of a pond where it spreads out to cover an area of 40cm2. Assume that the oil film has a uniform thickness equal to the diameter of an oil molecule. Calculate the diameter of an oil molecule and express your answer in scientific notation to an appropriate number of significant figures."

I have read a previous post where this question has been answered but I'm still unsure of how you calculate this. Its worth 4 marks in my assignment. The only thing I have so far is volume divided by area equals thickness. Any help would be appreciated!

You have been given the formula. You know the volume and you know the area. All you have to do is perform the division.

As I told you earlier, either you or the person who stated the problem has incorrectly given the area as 40 cm^2, when it should be 40 m^2. You are going to have to decide which area to use. Then state the answer to one significant figure. Personally I would use two signficant figures since the fist figure is will be 1, but that is another decision you will have to make.

The figures we were given are 0.05cm3 for volume and 40cm2 not metres2. I have calculated 0.05cm3 divided by 40cm2 resulting in 1.25-03 cm3. As 40cm2 is expressed to one sig fig then the answer would be 1-03cm3? I was unsure if there was anything else I needed to do as it is worth 4 marks and the calculation seems a bit simple

I'm not too sure which answer to go with : 1.25x10-3 1.2x10-3 1.3x10-3 or 1.0x10-3

The area of the droplet should

have read 40 m3 not 40 cm3 it was a typo error hope this helps

It was a typo in the assessment booklet?

40 m^3 is not an area. You seem to have made another typo.

40 m^2 (square meters) is an area

I'm confused now, it definitely says 40cm2 in my assessment booklet! Does this mean that my answer is wrong?

0.05cm3 divided by 40cm2= 1.25x10-3

I rounded to 2 s.f giving 1.3x10-3

You were given an incorrect number for the area. A typical diameter of an oil molecule is 1x10^-7 cm, not 1x10^-3 cm.

The answer would NOT be stated in units of cm^3. That is a unit of volume. It is important for you to realize that dividing cm^3 by cm^2 gives an answer with units of cm.

I do not know if the test expects you to give the wrong answer using its wrong data on the area. You will have to take your chances.

So far I have 0.05cm3 divided by 40cm2=1.25x10-3cm2 which I have rounded to 2sf, 1.3x10-3cm2. Would 40cm2 be an typo error in my booklet?

Yes. The person who made up the question erred, that is the kindest way of putting it.

ok, if I write 0.05cm3 divided by 40cm2 = 1.3x10-3cm. The centimetres in the answer would not be cm2 or cm3 just cm? I think I am just confusing myself even more here! I am teaching myself this so it's a bit difficult to get help when I'm stuck.

correct. cm^3/cm^2= (cm*cm*cm)/(cm*cm) = cm

thanks so much