pH related questions?

Question

Just a few questions. I need to learn how to solve them.

1. The pH of a solution is 2.1, what is the [OH-]?

2. When [H+] = 4.28 x 10-10 M, what is pH and pOH?

3. The pOH of a solution is 11.5, what is the pH and [OH-]?

4. If the hydrogen ion concentration of a solution is 9.58 x 10-5 M, what is the pOH and hydroxide ion concentration?

5. The hydrogen ion concentration of a solution is 6.25 x 10-12 M, what is the hydroxide ion concentration, pH, and pOH?

The main thing I don't understand is the negative logarithm thing. Just an easy to understand explanation would be a life saver.

Answer 1

1. pH + pOH = 14
pOH = 14 - pH = 14 - 2.1 = 11.9
[OH-] = 10^-11.9 = 1.3 x 10^-12 M

2. pH = - log [H+] = - log 4.28 x 10^-10 = 9.37
[H+] [OH-] = 1 x10^-14
[OH-] = 1 x 10^-14 / [H+] = 1 x 10^-14 / 4.28 x 10^-10 = 2.34 x 10^-5 M

3. [OH-] = 10^-11.5 = 3.16 x 10^-12 M
[H+] = 1 x 10^-14 / 3.16 x 10^-12 = 0.00316 M
pH = - log 0.00316 = 2.20

4. [OH-] = 1 x 10^-14 / 9.58 x 10^-5 = 1.04 x 10^-10
pOH = - log 1.04 x 10^-10 = 9.98
pH = 14 - pOH = 4.02

5. [OH-] = 1 x 10^-14 / 6.25 x 10^-12 = 0.0016 M
pOH = - log 0.0016 = 2.80
pH = 14 - 2.80 = 11.2

When we have to solve log of a number which is less than one as 0.1 we get :
log 0.1 = -1
pH = - log 0.1 = - ( - 1 ) = +1
I hope this will help you

Answer 2

Here are a couple of things you MUST understand to get the whole concept of pH:

A) First, in pure water, pH = 7, right? How do we get that number? It comes from the Dissociation Constant for water, Kw = [H+]*[OH-]/[H2O]
but in this equation, [H2O] = 1 by definition, so:

Kw = [H+]*[OH-] = 10^(-14)

But in pure water:
H2O <===> H+ + OH-, so [H+] must equal [OH-], thus:

Kw = [H+]*[H+] = [H+]^2 = 10^(-14), so [H+] = 10^(-7)

Now, the definition of pH is: pH = -log[H+

so, for pure water, pH = -log{10^(-7)},

The base 10 logarithm of 10^(-7) = -7 (that is the power to which 10 is raised), so the negative of -7 = 7, and that's how you get the pH value.

Okay, so now you get how we calculate pH. pOH is exactly the same, just looking at the hydroxide ion. So, in pure water, pOH = -log[OH-] = -log{10^(-7)} = 7

B) Here's the next critical trick:

pH + pOH = 14 (always, always,always!!! Every single time!)

So, if you know a value for pH, you can easily add or subtract to get the value for pOH, then use the logarithms to get [OH-], etc.

pH = 14 - pOH
pOH = 14 - pH

Okay, on to your examples:

1) pH = 2.1, therefore pOH = 14 - 2.1 = 11.9
[OH-] = 10^(-11.9) = 1.26 x 10^(-12) moles/liter

2) [H+] = 4.28 x 10^(-10)
pH = -log{4.28 x 10^(-10)} = 9.37
pOH = 14 - 9.37 = 4.63

3) pOH = 11.5
pH = 14 - pOH = 14 - 11.5 = 2.5
[H+] = 10^(-2.5) = 3.16 x 10^(-3) moles/liter

Do you think you can get 4 and 5 from there?

Answer 3

It's a mathematical operator like multiplication. It means what to do with the number.
start with
10^1=10
10^2=100
10^3=1000
10^4=10000
log (10^4)=4
log (10000)=4
log (0.1)=-1
log (.01)=-2

so log of 4.28^-10 ends up being about -9.368
since pH = -(log 4.28^-10 )
-(-9.368) or 9.368 is the pH

now if they give you the pH
9.368, just negative it, then put it in 10^X
10^-9.368 =4.28*10^-10

Answer 4

well... p is a mathematic function that stay for -log. It's just definition. So, pH is -log [H+].

exp is the inv. funicon of log. so, in u wanna find [H+], hust calculate 10 ^log[H+]. i.e.: if pH= 2 >>> [H+] = 10^-2

To calculate pOH just : 14-pH. to calculate [OH-] just consider how much is pOH and do the same of [H+]

chemist use -log coz when u calculate log of a number minor of 0, log u find is negative, i.e.: log0.1 =-1

so if u use -log, u can just use positive number coz: - log0.1 = -(-1) = 1