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If you were heating the solutions without covering the flasks, the evaporating water left the flask concentrating the acid, hence why you obtained a lower pH reading. Commercial vinegar is usually a 5% solution of acetic acid (CH3COOH).


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Just to add on, the pH can be greater than 14 if the temperature of the solution is low, in which case the water dissociation constant will decrease. If a given solution had a really high pH that is close to 14 at room temperature then it would definitely increase to a value beyond 14 if the temperature of the solution were to decrease.


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This question and the one referenced by Mithoron address NH$_4$HF$_2$ as an entity in itself which must be examined as a whole. It is easier conceptually to rewrite the formula for the compound as NH$_4$F.HF. Consider adding NH$_4$F to water: you get a pH near 7. In a separate container, add HF to water (~0.1 M); you get a pH ~1 https://ehs.princeton.edu/...


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You did not mention what kind of waste water are you talking about. Is it from a textile industry, paper mill, leather factory? Waste water is not a "compound", it can be any junk and its pH can vary over several orders of magnitude! As I have stated before, there is nothing fundamental about pH. It is a scale of convenience. It can be negative and ...


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"Hydronium" content can be easily determined by simple acid base titration. Keep in mind that there is nothing fundamental about pH. It is just a convenient scale for expressing hydrogen ion concentration. For example, a kilogram is a recent invention for expressing mass measurement, it does not mean that ancient people did not weigh anything ...


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Aluminum pieces are always covered by a thin, waterproof and nearly monomolecular layer of alumina, or aluminum oxide $\ce{Al2O3}$. This layer prevents the aluminum metal to get in touch with water. Because, pure and non-protected aluminum would immediately react with water according to : $$\ce{2 Al + 6H2O -> Al(OH)3 (s) + 3 H2 (g)}$$ This reaction ...


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Electrolysis of sodium chloride solution produces results at each electrode. At the cathode, water is reduced to $\ce{OH-}$ and makes the solution more alkaline. Copper hydroxide is a blue, bulky precipitate, feebly acidic, and soluble in concentrated alkali hydroxide solutions. So if you got copper ions into solution at the anode, you could form some cupric ...


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Solutions of aluminum sulfate are highly acidic. The Merck Index states that at a concentration $0.2 M$, the pH is $3.0$.


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Maybe a numerical example will help you. Let's start from pure water, with the following concentrations :$\ce{[H+] = [OH-] = 10^{-7} M}$. Now we will suppose you add $\ce{10^{-7}} mol$ $\ce{HCl}$ in one liter of this water. Suddenly, the concentration of $\ce{H+}$ should double. This is not possible in the long run. A part $a$ of these supplementary $\ce{H+}$...


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It is a very open ended question, but first three choices will cause errors. Regarding your pH issue, you are right, it is multiplicative error. Take the example of phenolphthalein, as you say that its color (=absorption spectrum) is different in acidic or basic medium. So pH change can shift the lambda max of analytes, hence the cause of error. ...


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I like what Maurice said "When nobody understands a scientific phenomena, we give it a name. Here the name is overpotential. Here the hydrogen is said to have a big "over potential" on mercury cathode. That is a bright and remarkable way to hide our ignorance." This is little on the extreme side but people have spent their life on ...


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