Acid and Base Flashcards
Bronsted-Lowry definition of acid base behaviour
Acid is defined as a substance that can donate a proton
Base is defined as a substance that can accept a proton
Calculating pH =
of STRONG ACIDS
-log[H+]
where [H+] is the concentration of hydrogen ions
Always give pH values to 2 d.p
Difference between strong and weak acids
Strong acids dissociate completely
Finding [H+] from pH
10^-pH
Expression for Kw =
[H+] x [OH-]
as [H2O is much bigger than the concentrations of the ions, we assume its value is constant and make a new constant Kw
Value of Kw for all aqueous solutions at 25C
10^-14 mol2dm-6
concentration volume formula
C1V1 = C2V2
pH of STRONG BASES
work out [H+] using [H+] = Kw / [OH-]
then use -log[H+]
As temperature increases equilibrium moves to the right to oppose the increase in temperature
therefore [H+] and [OH-] increases
so Kw increases
and pH increases
Calculating the pH of pure water
in pure water [H+] = [OH-]
therefore Kw = [H+]^2
so [H+] = √Kw
What is Ka
acid dissociation constant
Ka = (formula)
[H+][A-] / [HA]
pKa =
-logKa
Ka =
10^-pka
units of Ka
moldm-3
the bigger the value of Ka
the stronger the acid
the smaller the value of pKa
the stronger the acid
In a solution of a weak acid in water, with nothing else added
[H+] = [A-] [HA] = [HA] initial
WEAK ACIDs in aqueous solution with nothing else added
Ka =
[H+]^2 / [HA]
How to find the pH in reactions between weak acids and strong bases
- calculate moles of HA
- calculate moles of OH-
- calculate moles of excess HA or OH-
then use appropriate formulas
if mol HA = OH-
4. pH = pKa of weak acid
HA
weak acid
Buffer solution
A solution which resists change in pH when a small amount of acid or base is added
