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# Definition of pH

Introduction to pH and the pH scale. Examples of calculating pH of pure water, bleach, and orange juice.

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• Is there a maximum or minimum pH?
• But on a normal ph scale, the lowest is 0 and the highest is 14, with a 7 or rear being neutral
• Thank you Mr. Khan, I learn a lot from your videos.
I've got a question. In the video, the pH of the orange juice was 4. Then does that mean that in a cup of orange juice, there are 1x10^(-4) moles/liter of Hydrogen? does 1x10^(-4) refer to the amount of Hydrogen in the orange juice?
Thank you :D
• It means, in every 1 litre of orange juice, there are 10^(-4) moles of H+.
Which means 10^(-4) x 6.02x10^23 = 6.02x10^19 H+ ions. (approx)
• Why is the conventional pH scale from 0 to 14 and not from 1 to 14? How can you raise a number to an exponent such that you get 0?
• It is a logarithmic scale, not an exponential scale. A pH of 0 corresponds to an H⁺ concentration of 1 M.
• How does bleach have less hydrogen ions than water?Is most of it then hydroxide?
• bleach is basically a solution of sodium HypoChlorite, or NaClO. This is a salt that contains the anion of the weak acid HClO, hypochlorous acid, and the cation of the strong base, NaOH. Because NaOH is a strong base, it fully dissociates into Na+ ions and OH- ions. However, the HClO is weak and does not fully dissociate. Therefore, there is more OH- ions inside the solution and less H+ ions. This makes a Basic Solution.
• I got confused how we can calculate -log[proton] on the calculator.
Some people says log on calculator always has base 10, but some other people say the base is e. Isn't the log base e is equal to ln?
Same to writing, I saw physics teachers write log and they say in physics, its base is e, but my chemistry teacher says the base is 10.
How can we know whether the base is 10 or e?
• The difference is:
*If you just have "log" (without a base underneath) it means it is base 10, it is implied.
*On the other hand if you have "ln", it is implied that you have a base of "e" (Euler's constant).

Logarithms are largely used in a lot of different fields of study, and their base depends on the phenomenon that you are referring to (basically, you use the one that helps you the best to explain that certain phenomenon), that's why sometimes you might even find bases even different than e or 10, although these are the ones that are used the most.
• how do you measure the hydrogen ion concentration in a liquid? with a microscope?
• If you want an accurate measurement you will typically use a device called a pH meter§.

There are also chemicals called pH indicators that can be used.

These chemicals are often bound onto small strips called pH paper than change color to give you an estimate of the pH.

Note that only a very few microscopes (e.g. Scanning tunneling electron microscopes — STMs) are capable of seeing individual atoms and even they can't resolve a single proton (hydrogen ion).

Also, STMs work best with the sample in a vacuum and require the atoms to be attached to a surface, so you couldn't look at ions within a liquid sample.

Finally, even if you could make this work you would have to count thousands or millions of molecules and ions to get an accurate concentration and that would be very challenging!

Does that help?

§Note: If you want to know the details of how pH meters work I recommend these sources:
https://www.explainthatstuff.com/how-ph-meters-work.html
https://en.wikipedia.org/wiki/PH_meter (gets very technical)
• Consider a given amount of water. If we add n equivalents of an acid ((H sub y) x), y being the number of hydrogen ions in one molecule of the acid and x being some other element or ion...
In this case, the amount of H+ ions in the water will increase because of the donation of H+ ions by the acid. So, we can calculate the pH of this acid. But what if this acid was not dissolved in water, how would we calculate the pH of the acid in that case because the acid will not donate any protons to anything because of the absence of its solution (or in other words because of the absence of water) and hence the concentration of H+ ions would not change anywhere?
Is the pH not defined in that case, or is it just that we cannot calculate it but it does exist, or can we calculate it (if yes then how, if i may ask)?
Thank- you and sorry for the question being so long.
• Your suspicion is correct - pH is only relevant to solutions, though they apparently do not need to be aqueous.

For example you can calculate the pH of a solution where ammonia is the solvent - in this case protons are taken up by ammonia molecules to form ammonium. This is analogous to formation of hydronium ions in aqueous solutions.

see:
http://chemistry.stackexchange.com/a/7000
• : Stomach acid is just HCl right?
• That is correct. HCl (hydrochloric acid) is the acid formed by the stomach during digestion.
• Who created the mathematical term molar, and why did he/she make it so exact?
• The term "molar" in chemistry was not created by a specific individual, but rather emerged from the development of the field over time. The concept of the mole, which is the basis for the term "molar," was introduced by the chemist and physicist Amedeo Avogadro in the early 19th century.

Amedeo Avogadro proposed Avogadro's law, which states that equal volumes of gases, at the same temperature and pressure, contain the same number of molecules. He hypothesized that gases could be measured and compared based on the number of molecules they contained rather than their mass.

The term "mole" was coined in 1897 by the German chemist Wilhelm Ostwald, who derived it from the German word "Molekül" (molecule). It was later adopted internationally to represent the unit of measurement for the amount of substance in chemistry.
Hope it helps you.
• If given 1M solution of HCL then what will be its pH(0?) and what does negative pH mean??