Oceans 2: pH Calculations Flashcards
Formula for pH
pH = -log [ H+ ]
Formula for [ H+ ]
[ H+ ] = 10^-pH
What do [ ] represent
Concentration of
How many decimal places should pH always be given too
2
How are concentration of H+ ions and concentration of acids linked for STONG acids only
[ H+ ] = [ acid ]
For strong acids only
- 4 moles of H2 SO4 = ??? moles of H+ ions
2. 0.2 moles of HCl = ??? moles of H+ ions
- ) 4 moles of H2 SO4 = 8 moles of H+ ions
4 x 2 ( H molecules )
2.) 0.2 moles of HCl = 0.2 moles of H+ ions
Equation for dilution
( works in same way for both strong acids and strong bases - used to calc
[ H+ ] or [ OH- ] where both = C2 for example
•C1 V1 = C2 V2
- C1 & V1 = initial concentration & volume
- C2 & V2= new concentration & volume
How does equation for dilution work
- Usually calculating C2 = [ H+ ]
- Then plug conc of H+ ions into pH formula to calc pH
Give symbol of ionic product of water
Kw
What equilibrium equation is the ionic product of water formed from
H2O <=> H+ + OH-
For the ionic product of water equilibrium equation below which direction is endo / exo
H2O <=> H+ + OH-
Forward reaction = ENDOthermic
Reverse reaction = EXOthermic
Give the equation of for Kw
•Kw = [H+] [OH-]
And
- Kw= Kc [ H2O ] = a constant
How does temperature effect the pH of water and neutrality of water
Increase temp = increase pH ( number actually gets smaller = increase acidity )
BUT remains neutral as still [H+] = [OH-]
Why does increasing temp increase pH of water ( number gets smaller = increase acidity )
Increase temp
= equilibrium shift right ( forward endo )
= [H+] and [OH-] increases
= Kw increase
= pH increase
( but neutrality remains the same )
Definition of neutral
[H+] = [OH-]
In pure water what is true for the value ( or equation ) of Kw
Kw = [H+]^2
Because in pure water:
[H+] =[OH-]
Kw increase with increasing temp.
When temp increases water
A. Remains neutral
B. Dissociates less
C. Becomes acidic
D. Becomes alkaline
A. Remains neutral
How do u calculate the pH of stong base ( Method 1 ) ie. using [ OH - ]
- [ H+ ] = Kw / [ OH-]
2. pH = -log[ H+ ]
How do u calculate the pH of stong base ( Method 2 ) ie. using [ OH - ]
- pOH = -log[ OH- ]
2. pH = 14 - pOH
How do u calculate [ OH - ] in a strong base given the pH ( Method 1 )
- [ H+ ] = 10^-pH
2. [ OH- ] = Kw / [ H+]
How do u calculate [ OH - ] in a strong base given the pH ( Method 2 )
- pOH = 14 - pH
2. [ OH- ] = 10^ -pOH
What are the 5 steps for calculating pH of mixtures of strong acids and strong bases
- Calculate moles of H+
- Calculate moles of OH-
( moles = conc x vol( dm^3) )
- Calculate moles of XS H+ OH-
( H+ XS = H+ - OH-)
( OH- XS = OH- - H+ ) - Calculate conc of XS [ H+ ] or [ OH- ]
( conc = mol / ( total vol in dm^3 ) ) - Calculate pH
( pH = -log[ H+] )
( pOH = - log[ OH- ] —> pH = 14-pOH )
What does Ka stand for
Acid dissociation constant
Give the the 3 formulas of Ka
( hint first one derived from equation of weak acid ( equilibrium equation) :
HA <=> H+ + A- )
- Ka = [ H+ ] [ A ] / [ HA ]
- Ka = [ H+ ]^2 / [ HA ]
(only holds for weak acids in aqueous solution with nothing else added) - Ka = 10^-pKa
Give formula for pKa ( a weak acid expression ) ( similar to formula of pH )
pKa = - log Ka
What are the 2 assumptions for this this Ka formula:
Ka = [ H+ ] [ A ] / [ HA ]
- [ H+ ] = [ A- ]
Therefore Ka = [ H+ ]^2 / [ HA ] - [ HA ] is what’s given in the question
( so little has dissociated = we assume concentration has remained the same )
What does a bigger Ka value tell you about an acid
Bigger Ka value = stronger acid
What does a smaller pKa value tell you about an acid
Smaller pKa value = stronger acid
