Explain the term pH value

Answer 1

A pH value tells us how much H+ is in a solution. It can be defined by:

pH = -log[H+] (log is to base 10) (Also note, square brackets are used to denote concentration values)

So...if we have a strong acid, we get 100% dissociation, that's why it's "strong". So for every acid molecule, we get one H+ flying off into the solution.

So our equation, for a strong acid, can be

pH = -log[Acid]

So, let's ignore weak acids because they're a bit more complex and they don't serve to answer the question, really. One of our definitions of an acid is it's ability to donate H+.

So the more H+, the more acidic. And, using our equation, the higher H+ concentration, the lower pH calculates to be.

And we know a low pH values means acidic. So that's cool. And try it yourself. Get a calculator and do -log[Acid] calculations to get a feel for things. Try it with [Acid] = 0.1 and then =0.0001 and see the pH change.

In the first example, 0.1 here means the H+ conc is 0.1 mol dm-3.

In the second, 0.0001 means the H+ conc is 0.0001 mol dm-3.

So you'd expect a higher pH for the second, since H+ conc is lower and so is LESS acidic.

And that's what you'll get. Also note how the pH value gives you a rough estimation on where the decimal point is. It's a point I always include because it's nice (and valuable) to be able to look at a pH value and instantly have a rough idea of what it's telling you.

So while pH of 1 will give you 0.1 a pH of 2 would give 0.01 and so forth. So glancing at a pH value can tell you the concentration of H+ in your solution.

If my water is pH 7, I can say it has roughly 0.0000001 mol dm-3 H+ concentration.

So, in short: pH just tells us the free hydrogen cation concentration in a solution. It's lower for higher concentrations. Some things are more acidic because they give up the H+ easier than other things. Like HCl is a good acid because the H is fairly easily removed. Water is less acidic because it doesn't dissociate quite so readily.

The pH just tells us, for any given solution, how much H+ is in it.