VERY GOOD CHEMISTRY QUESTION!!!!! please help (1 Viewer)

[soisorce26]

hello all, i've given this question to alot of people, and yet noone has actually got an answer im content with. please help, its a past trial Q from SYDNEY TECHNICAL HIGH

A chemist is supplied with 0.010 mol/L solutions of 4 monoprotic acids (L,M,N, adn P) and measures the pH of each using a pH meter and data logger. The values are 4.2, 6.1, 2.0, and 2.7 respectively

(a) The chemist repeated his pH measurements ten times after lunch and found that the pH of each solution was now 0.1 pH units higher. Explain the probable cause of this change and thus state how the chemist could improve the reliability of his measurements

 

[Trebla]

How about this one:
Water may have evaporated from the solutions over time. The acid ionisation reaction is in equilibrium, hence as water concentration decreases, the equilibrium will shift to make less production of hydronium ions, hence lowering its concentration and increasing pH.
E.g. take acetic acid:
CH₃COOH + H₂O <==> CH₃COO^- + H₃O⁺
when we lose water through evaporation, equilibrium shifts left, producing less hydronium ions. Hence a lower concentration of hydronium/hydrogen ions leads to increased pH when applying pH = - log₁₀[H⁺]

Reliability can be increased by covering/sealing the beaker to prevent water escaping.....etc.....repeat experiments.....etc

 

[soisorce26]

The acid ionisation reaction is in equilibrium, hence as water concentration decreases...

umm i had the idea that equilibrium could only be achieved in a closed system. and wat if teh acid was strong? its not in the equilibrium state

 

[brenton1987]

How about this one:
Water may have evaporated from the solutions over time. The acid ionisation reaction is in equilibrium, hence as water concentration decreases, the equilibrium will shift to make less production of hydronium ions, hence lowering its concentration and increasing pH.
E.g. take acetic acid:
CH₃COOH + H₂O <==> CH₃COO^- + H₃O⁺
when we lose water through evaporation, equilibrium shifts left, producing less hydronium ions. Hence a lower concentration of hydronium/hydrogen ions leads to increased pH when applying pH = - log₁₀[H⁺]

Reliability can be increased by covering/sealing the beaker to prevent water escaping.....etc.....repeat experiments.....etc

But if the beaker is exposed then CO₂ will dissolve into the water and form H₂CO₃, which will partially dissociate into HCO₃^- and H₃O⁺. Which could counteract the evaporation of the solvent.
You would have to do some calculations to see if it would actually affect anything.

 

[soisorce26]

<tt>My suggestion is there must be a change in temperature in the laboratory conditions that would cause the equilibrium to shift to the left and lowered the concentration of H+ and a slight increase in pH.
The reliability can be improved by measuring the pH under exactly the same condition of temperature and concentration using a well calibrated pH probe.



this question drives me nuts!!! maybe il skip it...
</tt>

 

[xiao1985]

pH of acids is temperature dependent. so you are right.... though the influence will be rather small.
however, evaporation of water seems more plausible...

 

[Trebla]

umm i had the idea that equilibrium could only be achieved in a closed system. and wat if teh acid was strong? its not in the equilibrium state

The system is initially closed when they are stored in containers. Once they are exposed for testing, the equilibrium is disturbed, hence changes occur.
It does not matter whether the acid is strong or not, the ionisiation is still in equilibrium. It's just that with strong acids the equilibrium favours the forward reaction waaaayyy moreso than the reverse; so Le Chatelier's principle still applies...

It surely has something to do with the equilibrium in the ionisation. I mean, the suggestions above all refer to it and Le Chatelier's principle...

 

[kony]

1. original measurements were unreliable. (only made once)
2. pH meter not specified to have been properly prepared / calibrated.

 

[Trebla]

1. original measurements were unreliable. (only made once)
2. pH meter not specified to have been properly prepared / calibrated.

Ah yes....retreat into the good old "there's something wrong with your results idiot!!!!"

 

[CharlieB]

seen something similiar is a neap.
consider the equation of the self ionization of water, and whether it is a endothermic or exothermic reaction.

 

[Trebla]

seen something similiar is a neap.
consider the equation of the self ionization of water, and whether it is a endothermic or exothermic reaction.

I don't see how self ionisation of water would help. Whichever way the equilbrium would go for this, it would still be neutral.
I don't think the temperature would be a factor, because if this was carried out in a lab, wouldn't the temperature be relatively stable?

 

[wrxsti]

pH increase causes include:

i) Water evaporates (if acid is aqueous) meaning the molar concentration of acid increases and hence the pH.
ii) If exposed to air, CO2 will dissolve into it forming carbonic acid (H2CO3) which will make it slightly more acidic - especially since the experiment was repeated so many times - the immersing of a pH probe mixes the air with the acid.
iii) Temperature shifts equilibrium in a certain direction meaning the once acid ions become acid molecules (de-ionised)
iv) pH probe may not have been washed from previous dealing with acids - hence after repeating the experiment 10 times, there was a lot of inproper mixing of chemicals
v) A student went into the lab while the scientist went out to lunch and added more acid to each beacker :]
vi) The scientist ate lunch that contained an acidic substance, like vinegar, and dipped his hands accidentally in the acids afterwards, increasing their pH slightly :rofl:

To increase reliability, the scientist would control humidity, temp, breeze, UV exposure, etc. Record results in a table to minimise error when plotting results.

chem nerd

 

[ssejamafone]

pH increase causes include:

i) Water evaporates (if acid is aqueous) meaning the molar concentration of acid increases and hence the pH.
ii) If exposed to air, CO2 will dissolve into it forming carbonic acid (H2CO3) which will make it slightly more acidic - especially since the experiment was repeated so many times - the immersing of a pH probe mixes the air with the acid.
iii) Temperature shifts equilibrium in a certain direction meaning the once acid ions become acid molecules (de-ionised)
iv) pH probe may not have been washed from previous dealing with acids - hence after repeating the experiment 10 times, there was a lot of inproper mixing of chemicals
v) A student went into the lab while the scientist went out to lunch and added more acid to each beacker :]
vi) The scientist ate lunch that contained an acidic substance, like vinegar, and dipped his hands accidentally in the acids afterwards, increasing their pH slightly :rofl:

To increase reliability, the scientist would control humidity, temp, breeze, UV exposure, etc. Record results in a table to minimise error when plotting results.

lol. i think the answer is (v). =D

But i would seriously say that (i) and (ii) are the most probable causes.

 

[CharlieB]

I don't see how self ionisation of water would help. Whichever way the equilbrium would go for this, it would still be neutral.
I don't think the temperature would be a factor, because if this was carried out in a lab, wouldn't the temperature be relatively stable?

Self-ionisation of water:

2H20(l) <=> H30+(aq) + OH-(aq)


Reaction is endothermic. Increased temperature would shift reaction right. Producing more H30+.
pH decreases, but it doesn't mean its more acidic as the hydroxide concentration is also higher by the same amount.

but yeh i think errors such as the cleaning of the probe seems like a more suitable answer.

 

[Trebla]

i) Water evaporates (if acid is aqueous) meaning the molar concentration of acid increases and hence the pH.

Um...not sure what you meant, but if concentration of acid increases the pH DECREASES, not increases. You have to consider the equilibrium argument which is more appropriate.

 

[mitochondria]

pH increase causes include:

i) Water evaporates (if acid is aqueous) meaning the molar concentration of acid increases and hence the pH.
ii) If exposed to air, CO2 will dissolve into it forming carbonic acid (H2CO3) which will make it slightly more acidic - especially since the experiment was repeated so many times - the immersing of a pH probe mixes the air with the acid.
iii) Temperature shifts equilibrium in a certain direction meaning the once acid ions become acid molecules (de-ionised)
iv) pH probe may not have been washed from previous dealing with acids - hence after repeating the experiment 10 times, there was a lot of inproper mixing of chemicals
v) A student went into the lab while the scientist went out to lunch and added more acid to each beacker :]
vi) The scientist ate lunch that contained an acidic substance, like vinegar, and dipped his hands accidentally in the acids afterwards, increasing their pH slightly :rofl:

To increase reliability, the scientist would control humidity, temp, breeze, UV exposure, etc. Record results in a table to minimise error when plotting results.

Lol! That is so true In fact, apart from ii), v) and vi) are probably the best answers in this entire thread ^^;

No offense to the original poster - but, this is really not a very good HSC chemistry question. I'm sure they're looking for answers like i), ii) and iii) but in reality those factors are fairly trivial.

In fact, I did some rough calculations And.. You need at least ~40% volume reduction to get a 0.1 change in pH at that concentration... And.. Temperature.. Pffft. CO₂ is the most sensible.. But, I don't think it'd make that much of a difference Besides, the information given isn't sufficient to make any sensible answers regarding CO₂

 

[polyspaston*]

Perhaps the acids are hygroscopic so they absorbed water. As a result, the hydrogen ion concentration decreased, which was then measured as a rise in pH.

 

[jlnWind]

^^^ Haha WTF hygroscopic... sounds... scary. lol


I'm not so sure about the explanation, i think it intended it to be open ended
like thats why they increase is 0.1 pH is fairly small.
I really liked Trebla's solution, creative haha i think they would've marked that correctly.

But i think i agree with mitochondria. The question is not really good at all. Especially the part where it asks "state how the chemist could improve the reliability of his measurements".

I think some of you may have misinterpreted the meaning of reliability.
Reliability refers not to the accuracy so much as the reproducibility. Accuracy is a measurement of the percentage error from an expected result. Something can be reproducible but not accurate. Conq Chem defines reproducibility as "the ability to get the same result when we repeat the experiment several times"

From what I can see, the guy repeated the experiment 10 times and got 0.1 increase (the whole time assumed), indicating a set of very reliable and repeatable measurments/ results.

Please correct me if i'm wrong, i dont wanna stuff up my hsc LOL

 

[brenton1987]

^^^ Haha WTF hygroscopic... sounds... scary. lol

Its something that will absorb water from the surrounding environment.

 

[jlnWind]

^^ thanks.
Does that mean it can be used to define a dehydrating substance
H2SO4?