Concepts
pH Scale
So what exactly is pH? Well to start is stands for potential Hydrogen, but it practical terms it indicates the strength of the acid or base. The pH scale goes from 0-14 with 0 being a very strong acid 14 being a very strong base. The breakdown of what the numbers mean goes like this:
So what exactly is pH? Well to start is stands for potential Hydrogen, but it practical terms it indicates the strength of the acid or base. The pH scale goes from 0-14 with 0 being a very strong acid 14 being a very strong base. The breakdown of what the numbers mean goes like this:
0-3 is a strong acid
greater that 3 but less than 7 is a weak acid
7 is neutral
greater than 7 but less than 10 is a weak base
10-14 is a strong base
greater that 3 but less than 7 is a weak acid
7 is neutral
greater than 7 but less than 10 is a weak base
10-14 is a strong base
It's important to remember the pH only indicates it strength, it doesn't determine it.
pOH Scale
pOH is very similar to pH but instead of measuring potential hydrogen it measures potential hydroxide. It's still indicating the strength but now the breakdown is as follows
pOH Scale
pOH is very similar to pH but instead of measuring potential hydrogen it measures potential hydroxide. It's still indicating the strength but now the breakdown is as follows
0-3 is a strong base
greater that 3 but less than 7 is a weak base
7 is neutral
greater than 7 but less than 10 is a weak acid
10-14 is a strong acid
greater that 3 but less than 7 is a weak base
7 is neutral
greater than 7 but less than 10 is a weak acid
10-14 is a strong acid
Converting between pH and pOH is incredibly easy. You just take the value you have and subtract it from 14 and it gives you the other one. For example if I had an acid with a pH of 2 and I wanted to find the pOH I would subtract 2 from 14. This gives me 12 and tells me the pOH! If I were to put this in an equation if would be:
14 - pH = pOH
14 - pOH = pH
14 - pOH = pH
[H+] and [OH-]
So what the heck is that? And why do you need to know it?
The brackets around the H+ and OH- means we're talking about concentration of those ions. That comes into play in the calculations. Normally when we talk about H+ ions we're talking about an acid, as we know an acid produces H+ ions according to Arrhenius' theory. Inversely when talking about OH- ions we're normally talking about bases for the same reason.
So what the heck is that? And why do you need to know it?
The brackets around the H+ and OH- means we're talking about concentration of those ions. That comes into play in the calculations. Normally when we talk about H+ ions we're talking about an acid, as we know an acid produces H+ ions according to Arrhenius' theory. Inversely when talking about OH- ions we're normally talking about bases for the same reason.
Calculations
Solving for pH
Now let's say that you want to figure out the pOH of an acid but all you have is its concentration of H+ ions, or you want to find out the concentration of of base but you only know its pH, how do you do that? Well there's a formula and here it is: pH = -log[H+] Let's use this formula to figure out the pH of an acid with a 0.1 molarity. The concentration of H+ is the same as its molarity so we know that [H+] = 0.1M Now we can plug that value into our equation to give us: pH = -log[0.1] We then put -log[0.1] into the scientific calculator to the right and we get a value of 1 for the pH. Our final answer is then pH = 1 |
Solving for [H+]
Now let's say you knew the pH of the acid was 1 but you wanted to know the concentration of H+ ions, what would you do? You can start with the same formula and put in the known pH.
1 = -log[H+]
You then multiply both sides by negative one
-1 = log[H+]
Now here is where it gets different. Next you are going to take the inverse of the log function and multiply that to the power of the negative pH. Trust me, it works. To plug that into your calculator you click the 10 to the x power button and fill x in with the negative pH. It should look like this
Now let's say you knew the pH of the acid was 1 but you wanted to know the concentration of H+ ions, what would you do? You can start with the same formula and put in the known pH.
1 = -log[H+]
You then multiply both sides by negative one
-1 = log[H+]
Now here is where it gets different. Next you are going to take the inverse of the log function and multiply that to the power of the negative pH. Trust me, it works. To plug that into your calculator you click the 10 to the x power button and fill x in with the negative pH. It should look like this
10-1 = 0.1
This gives you the concentration of [H+] ions at 0.1M which we know is what we wanted. Although 0.1 is a manageable number, some values for concentration can get very small, so for those we use scientific notation. For this the scientific notation would be 1 x 10-1. This can also be written as 1e-1 which is how you'll see it written on the quiz. This is also the way you would put it into your scientific calculator. That's it!
This gives you the concentration of [H+] ions at 0.1M which we know is what we wanted. Although 0.1 is a manageable number, some values for concentration can get very small, so for those we use scientific notation. For this the scientific notation would be 1 x 10-1. This can also be written as 1e-1 which is how you'll see it written on the quiz. This is also the way you would put it into your scientific calculator. That's it!
Solving for pOH and [OH-]
Sloving for pOH and [OH-] is very similar to solving for pH and [H+] except now you are looking at hydroxide ions. To solve for pOH and [OH-] you use the same formulas but switch out pOH for pH and [OH-] for [H+] like so:
pOH = -log[OH-]
All the steps for the calculations are the same.
Finding the unknown values
The last step is to make sure you're comfortable finding the value of the pH, pOH, [H+], or [OH-] based off of only knowing one value. Going from pH to pOH or vice versa very simple and just involves subtracting whatever one you do know from 14 to find the other. Click here to review that if needed. To go from [OH-] to [H+] is a little harder as you first must find out the pH or pOH.
You've done it all on this page! Now go try out your new skills on the quiz! Just click the button below and off you go!