21.2 The PH scale and strong acids/bases

Question 1

What is PH measured by

Answer 1

PH= -log10[H+]

Question 2

How does the PH scale work

Answer 2

The smaller the PH, the greater the concentration of H+ ions in solution

A difference in one PH number means a tenfold difference in [H+] so for example pH 2 has ten times as many H+ ions as pH 3

Question 3

How do you calculate PH of neutral solutions

Answer 3

. At 298k, kw = [H+aq][OH-aq]
which is 1 x 10^-14

So [H+] = 1 x 10^7
so if you put this into PH calculation

-log10(1 x 10^7)

Question 4

How does PH scale measure alkalinity

Answer 4

As H+ ion concentration decreases, OH- concentration increases

At 298k, if a solution contains more H+ ions than OH- ions, it is acidic, and vice versa.

Question 5

How do you calculate [H+]

Eg in a solution with pH 3

Answer 5

3= -log10[H+]
-3= log10[H+]

Take antilog of both sides
so 10^-3 is concentration of H+ ions

So concentration is 1 x 10^-3

Question 6

How do you calculate [OH-] in solution with pH 10

Answer 6

10 = -log10[H+]
-10=log10[H+]

so 10^-10 = [H+]

so concentration of H+ ions is
1 x 10^-10

We know that [OH-][H+] = 1 x 10^-14
so find OH- by dividing it

So [OH-] = 1 x 10^-4

Question 7

What are strong acids

Answer 7

Acids that dissociate completely in solution

Question 8

How do you find PH of a strong acid solution

Find PH of HCl in 1 moldm-3 HCl

Eg HCl —> H+ + Cl-

What about in 0.16moldm-3 of HCl

Answer 8

[H+] = 1moldm-3
log[H+] = log(1)
-log[H+] = 0
so PH of 1moldm-3 is 0.00

[H+] = 0.16
log[H+] = log(0.16) = -0.796
-log[H+] = 0.796

so PH is 0.8

Question 9

Why does PH of pure water decrease when temperature is increased

Answer 9

Kw is a constant at room temperature as
[H+]=[OH-] and it has value
1 x 10^-14

Forward reaction of equilibrium of water is endothermic so when temp of water is increased it will shift to the right.
This makes more H+ ions so PH decreases.
However this doesn’t mean the water is acidic because
[H+]=[OH-]

H2O <—-> H+ + OH-