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Topic: Determine electrolyte class/conductivity levels from graph  (Read 14382 times)

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Offline MathisFun

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Determine electrolyte class/conductivity levels from graph
« on: October 14, 2012, 01:51:34 PM »
Hello, the following is from a standard chemistry course at the university level. It deals with the behavior of electrolytes, in its many varied forms (strong, weak, non), in an aqueous solution. I posted my responses to the questions, not confident about it though.

The student used a conductivity probe to determine the amount of ions in an aqueous solution. Six different calibration points were made.

This is represented by the following link, which displays the graph, and the conductivity values used, below the link.
http://i46.tinypic.com/2gui8g3.jpg
k = conductivity, with its units being uS/cm
KCl = Potassium Chloride used as a conductivity standard

Distilled water, k = 0.0 uS/cm**
0.00400 M KCl, k = 578.0 uS/cm**
0.00800 M KCl, k = 1202.6 uS/cm**
0.0200 M KCl, k = 2949.6 uS/cm**
0.0400 M KCl, k = 5560.0 uS/cm**
0.0600 M KCl, k = 8426.9 uS/cm**
**0-10,000 uS/cm setting

--Looking at the conductivity probe calibration curve in the graph, what is the relationship between conductivity and the concentration of a strong electrolyte?

Based on the graph, it seems that the greater the concentration of a solution, the less diluted it is, would in effect, yield a higher value of conduction, representing a greater number of ions present in the solution. Since KCl is used, and it is a strong electrolyte like NaCl, I’m going to assume that 0.02000 M NaCl would yield a similar value as 0.0200M KCl above and in the graph.

My logic tells me otherwise, with the above statement. "Since KCl is used, and it is a strong electrolyte like NaCl, I’m going to assume that 0.02000 M NaCl would yield a similar value as 0.0200M KCl above and in the graph."

--In comparison, what would a graph of conductivity versus the concentration of a weak electrolyte look like?

If a weak electrolyte were to be used, it would fall somewhere below the line in the graph.

But the question for me is, can a weak electrolyte give off a reading that can confuse the person looking over the graph, and come across as being a strong electrolyte? I’m on the outs on how to determine what the electrolyte conductivity would be for a weak electrolyte as opposed to a strong one, especially if a set of unknown solutions were to be given, and charted on a graph. It becomes an issue of where is the point that separates a strong electrolyte from a weak one. Without knowing the substances involved, it generates more confusion on my part. Is it simply by viewing the solution, and seeing the compound fully dissolve into ions? 

--What would a graph of conductivity versus the concentration of a non-electrolyte look like?
If tap water or a non-electrolyte, such as, C12H22O11 (sucrose), were to be used, it would produce the same value as distilled water, with 0.0 uS/cm. Therefore, the electrolyte conductivity of a non-electrolyte, such as distilled water, will always be 0.0 uS/cm, and never chart beyond this value, staying fixed at (0,0) on the graph.

Offline Borek

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #1 on: October 14, 2012, 03:22:49 PM »
Thing that you seem to be ignoring is that the plot is perfectly linear.

Does the total concentration of ions in a weak electrolyte change linearly with the concentration?
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Offline MathisFun

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #2 on: October 14, 2012, 04:04:55 PM »
I think that the graph would still produce a linear line.

For example, if different concentrations of CH3COOH (acetic acid) were to be measured using a conductivity probe, much in the same manner, as the strong electrolyte, KCl (potassium chloride) above.

0.00400 M CH3COOH
0.00800 M CH3COOH
0.0200 M CH3COOH
0.0400 M CH3COOH
0.0600 M CH3COOH

It would be similar to the linked graph in post #1, http://i46.tinypic.com/2gui8g3.jpg
but its values would be less, thus producing a linear line leaning more towards the x-axis of the graph.

That's only if the substance is known. What if the substance was an unknown? Or, a set of unknowns? For example:
0.02000 M Unknown A
0.02000 M Unknown B
0.02000 M Unknown C

If I were to use a conductivity probe and immerse it into each unknown solution above, how do I determine, whether or not its electrolyte conductivity classifies it as being a strong or weak electrolyte? My only lead is to compare the unknown solution's conductivity readings with the readings of known solutions using equal amounts of molar concentration:

0.02000 M KCl (strong electrolyte)
0.02000 M CH3COOH (weak electrolyte)

With that said, would it be fair to conclude that if two strong electrolytes using equal amounts of molar concentration, in this case,
0.02000 M KCl
0.02000 M NaCl

would therefore, produce similar values within range of each other. With the same holding true, if two weak electrolytes, such as,

0.02000 M CH3COOH
0.02000 M H2CO3

being measured.

Offline Borek

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #3 on: October 14, 2012, 04:08:37 PM »
Do you know how to calculate pH of the weak acid solution? Say 0.01 M acetic acid if Ka is 1.8x10-5?
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Offline MathisFun

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #4 on: October 14, 2012, 04:19:29 PM »
No, we haven't been taught that material yet. I know that strong and weak electrolytes, both have separate classes for acids and bases as well. At the moment, the class is looking at it from a theoretical point, basing it on the charges. So, the pH level of the solution is not taken into context.

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #5 on: October 14, 2012, 04:23:49 PM »
Strange.

pH of the solution doesn't matter much, but knowing how to calculate it you would be able to easily check that concentration of ions present doesn't change linearly with the concentration of the substance and the plot would be different.
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Offline MathisFun

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #6 on: October 14, 2012, 04:53:41 PM »
Hmm... that is different. But, my conclusion about strong electrolytes and its concentration of ions corresponding to its concentration of substance, seems to hold based on the graph. Taking your input into account, would the concentration of ions in a weak electrolyte, when measured with a conductivity probe, produce erratic values when measuring it to different concentrations of the same substance, therefore, producing a non-linear graph? And if so, that is something, I did not expect, since I'm basing my conclusion on the graph.

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #7 on: October 14, 2012, 04:56:33 PM »
Not erratic, just non linear. You would get a nice curve.
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Offline MathisFun

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #8 on: October 14, 2012, 05:10:43 PM »
It still follows that a higher concentration of substance would produce a higher conductivity reading, but why would a set of weak electrolytes produce a non-linear line, while a set of strong electrolytes produces a linear one?

Are both these examples possible for a weak electrolyte being charted with increasing concentration?
http://i47.tinypic.com/pugjd.jpg
http://i46.tinypic.com/28vd7p1.gif

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #9 on: October 14, 2012, 05:32:48 PM »
The latter is more what I would expect - conductance growing slower than the concentration. The higher the concentration the less fraction of the substance is dissociated. In total there is more ions, but concentration of ions grows slower than the concentration of the substance itself.
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Re: Determine electrolyte class/conductivity levels from graph
« Reply #10 on: October 14, 2012, 05:55:20 PM »
So, in effect, the strong electrolyte produces a linear graph because the concentration of ions grows at a higher rate with each reading of higher concentration, due to the compound dissociating completely.

Since tap water conducts a small amount of electricity, would it therefore be considered a weak electrolyte and produce a non-linear graph as well?

For a non-electrolyte, no line would be produced. It would be marked as one point at the origin of the graph.

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #11 on: October 14, 2012, 06:10:53 PM »
Since tap water conducts a small amount of electricity, would it therefore be considered a weak electrolyte and produce a non-linear graph as well?

No, it just contains relatively low concentration of ions. Most of these are strong electrolytes, with the exception of carbonates.
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Re: Determine electrolyte class/conductivity levels from graph
« Reply #12 on: October 14, 2012, 06:30:09 PM »
I see. Since the conductivity of distilled water is, k = 0.0 uS/cm, no matter how much volume is used, it will always give a reading of, k = 0.0 uS/cm. But, the electrolyte conductivity of tap water would be beyond, k = 0.0 uS/cm, due to the small amount of ions present, right? If that were the case, up to what point does a reading beyond, k = 0.0 uS/cm, constitute a non-electrolyte?

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #13 on: October 15, 2012, 03:58:43 AM »
If that were the case, up to what point does a reading beyond, k = 0.0 uS/cm, constitute a non-electrolyte?

No such value, as conductivity is a function of several factors. For example strong electrolyte made of very large ions can have very low conductivity even at high concentrations.
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Offline MathisFun

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Re: Determine electrolyte class/conductivity levels from graph
« Reply #14 on: October 15, 2012, 05:25:56 AM »
... conductivity is a function of several factors. For example strong electrolyte made of very large ions can have very low conductivity even at high concentrations.

So, a high concentration of a strong electrolyte can produce a low conductivity reading. If I took my probe and measured it with a vial of an unknown substance (0.02000 M Unknown A) with equal volume and concentration, as a previously measured solution, in this case, the strong electrolyte, potassium chloride (0.02000 M KCl), and it returned a value range within the scope of the previously measured strong electrolyte (k = 2949.6 uS/cm), would it be safe to assume, that the unknown substance, then, classifies as a strong electrolyte?

But, with your statement taken into consideration, does an individual run the risk of mis-labeling an unknown solution as a strong or weak electrolyte after measuring it with the probe, because of your quote: "... strong electrolyte made of very large ions can have very low conductivity even at high concentrations." Thus, being labeled as a weak electrolyte?

Since a non-electrolyte has no range of values, is there a particular range for strong or weak electrolytes to help distinguish it?

Using the info provided in my first post, I'm assuming a strong electrolyte runs within the range of, k = 578.0-8426.9 uS/cm.

The non-electrolyte, distilled water, is usually used to prep a conductivity probe. Would another non-electrolyte, such as, C12H22O11 (sucrose), suffice?

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