Question

The effects of osmosis.
20 ml of water and 20 ml of sucrose solution was combined (to obtain a conc. of 0.2 moldm^-3) and poured into a petri dish. When I recorded the measurements of the potato strip (i.e every 15 mins), I got this:

Answer 1

Before I calculate the difference and the % change, I would like to know why the results are like that. Why does it increase and decrease?

Answer 2

it's possible that it's just due to an equilibrium being established. It could've absorbed more water than it should've so it diffused out.

Answer 3

I see. When I'm calculating the % change, what sign should I place infront of the value? '+' for an increase in length and '-' for a decrease in length. In this case...i don't know :/

Answer 4

Are you calculating the difference from the original or from the measurement just before?

Answer 5

The measurement just before.

Answer 6

hmm then yeah, i would just write a negative percent.

Did you make these measurements yourself? if so, are you doing error analysis on this?

Answer 7

I did the lab yesterday, along with my classmates...we had a bit of a problem though. The balance was a bit old...and inaccurate.

Answer 8

plus the knife was rusty and the girl that was cutting the strips (to 5cm) was a speedy.

Answer 9

hm that could very well play a part in it.

It's possible that the samples werent all the same length.. or that the person measuring them didn't do a good job.

I guess it's too late to tell you this now, but everytime you make a measurement you should write down a range, like 5.25 to 5.30 cm - always go a number after the smallest measurement of the ruler. This is for analog instruments, for digital instruments, they have an uncertainty measured by the manufacturer.

Answer 10

The 5.2 and 5.3 measurements could very well be within error margin, so no fluctuation actually occurred.

Answer 11

That could actually be it but now it's late. I'll have to mention the possible sources of error in the discussion to account for this. By the way, would salt be more effective than sucrose?

Answer 12

hmm i think so, they're much smaller, for one, so the diffusion occurs quicker.

Answer 13

You'd have to know the surface chemistry (porosity) of the cells; but it'd be easier to just actually do it.

Answer 14

okay.

Also, do I have to include any calculations (such as calculations involved in the preparation of various concentrations of sucrose solutions) in this lab?

Answer 15

hm, i have no idea, does your manual say so? I mean, you only made you solution, so you could.

\(Molarity=\dfrac{moles}{L_{solution}}=\dfrac{\dfrac{mass}{Molar~mass}}{L_{solution}}\)

Answer 16

okay, then, I will.

One more minor question. I have to plot a graph of conc. of sucrose against % change. When I'm calculating the % change and say, for example, I got a value of 1.9. Should I round it off to 2? or should I leave it as it is?

Answer 17

hm, it depends, are all your other numbers whole numbers? if not i would just leave it as 1.9

Answer 18

Nope, all are not whole numbers. I got 1.95 for one. What should I do with this?

Answer 19

hm i would round up, 1 decimal place is good enough.

Answer 20

Alright, by the way, thanks for your help!

Answer 21

no problem !