I am doing a investigation about electrical conductivity of acids and alkalis and the change in conductivity during neutralisation. I want to know what calculations can I do in my analysis???I have no idea and i desparately need some help!!!
oh and one more thing, are conductance and conductivity two different things?I was confused about this and i don't know whether my data is conductance or conductivity because the meter is too old and it only displays the unit mS.
I need to be a bit careful in answering questions about coursework becaus eof the rules about help. However, I can give some general help.
The main thing with conductimtric titrations is that pure water has a very low ability to conduct electricity because it has very few ions present. Acids and alkalis have a much better conductivity because they ahve lots of ions present.
Let's assume you are adding acid to an alkali. H+ + OH- --> H2O (sorry about the lack of suitable typefaces!) As you add the acid, it uses its H+ ions to soak up some of the OH- ions of the alkali. Once you are past the end point, the ions don't get used up anymore and so the conductivity increase.
So you could plot a graph of the conductivity against the volume of acid added and find wher there is a kink in the graph to show the end point.
Which board are you studying for? This does not appear on the spec with whcih I am familiar so I will need to check some details to make sure that I am giving good advice. I have a feeling (but only that, so don't quote me yet!) that conductivity is the actual measurement of how much current can flow through a particular solution and that conductance takes into account eh oncentration etc...a sort of "conductivity per mole". As I said, I will check and get back to you.
Long term coursework usually comes with some briefing notes etc so have another chack through that, if you have it.
Thanks for your help!but I am still uncertin about conductance and conductivity. I've checked quite a lot of websites and they said conductance and conductivity are two things but generally they didn't explain how to define them.The conductivity meter that I used only said mS as the unit so which is right?
I am on OCR salters for chemistry,and I chose this investigation myself becasue I thought it would be interesting,but it turns out quite confusing.Unfortunately I don't have any notes or whatsoever, all the things I know is from internet.I am currently doing the analysis for a [strong acid weak alkali] and a [weak acid strong alkali]conductimetric titration, can you give me some hints?
I have just dug out my university chemistry textbook (Physical Chemistry written by Peter Atkins, in case you want to reference it).
He doesn't use the word conductance in this book at all (but see later). I will paraphrase some bits of his book.
The conductivity of a solution is determined by measuring its electrical resistance.
This is done in a conductivity cell. The resistance of a sample increases with its length and decreases with its cross-sectional area:
Resistance is proportional to length of cell / cross-sec area.
The proportional constant in this equation is called the resistivity. So...
Resistance is equal to resistivity x length of cell / cross-sec area.
Conductivity is the inverse of resistivity.
conductivity = length of cell / (Resistance x cross-sec area).
This is difficult to measure accurately and so it is usual to standardise the conductivity cell with a known solution (often potassium chloride)
The conductivity of the standard solution = C / measured resistance of the standard solution. (C is called the cell constant)
This means that the conductivity of any solution is always given by C / its measured resistance (use the same C for all experiments).
Because the conductivity of a solution depends on the concentration of the ions, it is a good idea to calculate the molar conductivity. This is the conductivity of the solution / the concentration in mol per litre.
Strong electrolytes (include the strong acid and strong alkali in this category) are totally ionised and weak electrolytes (like the weak acid) are only partially ionised.
Because the weak acid is only partially dissociated, it has only very few ions in the solution and so its conductivity is much less than for a strong acid.
Here is the "later bit". In another book (Atkins again but working with Julio de Paula this time) conductance is defined as being the inverse of the resistance.
By the way, this makes it look as if wrong(as I said I might be!) in my original reply.
This is a big chunk of information to digest. Have a look at this and then get back to me.
Depending on where you want to take this, you might want to have a look at Kohlrausch's Law (do a google search).
Coursework can be very time-consuming so make sure you focus on the criteria that you are supposed to be aiming for.
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