Oceans 2: pH Calculations

Question 1

Formula for pH

Answer 1

pH = -log [ H+ ]

Question 2

Formula for [ H+ ]

Answer 2

[ H+ ] = 10^-pH

Question 3

What do [ ] represent

Answer 3

Concentration of

Question 4

How many decimal places should pH always be given too

Answer 4

2

Question 5

How are concentration of H+ ions and concentration of acids linked for STONG acids only

Answer 5

[ H+ ] = [ acid ]

For strong acids only

Question 6

1. 4 moles of H2 SO4 = ??? moles of H+ ions

2. 0.2 moles of HCl = ??? moles of H+ ions

Answer 6

1. ) 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

Question 7

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

Answer 7

•C1 V1 = C2 V2

• C1 & V1 = initial concentration & volume
• C2 & V2= new concentration & volume

Question 8

How does equation for dilution work

Answer 8

• Usually calculating C2 = [ H+ ]

- Then plug conc of H+ ions into pH formula to calc pH

Question 9

Give symbol of ionic product of water

Answer 9

Kw

Question 10

What equilibrium equation is the ionic product of water formed from

Answer 10

H2O <=> H+ + OH-

Question 11

For the ionic product of water equilibrium equation below which direction is endo / exo

H2O <=> H+ + OH-

Answer 11

Forward reaction = ENDOthermic

Reverse reaction = EXOthermic

Question 12

Give the equation of for Kw

Answer 12

•Kw = [H+] [OH-]

And
- Kw= Kc [ H2O ] = a constant

Question 13

How does temperature effect the pH of water and neutrality of water

Answer 13

Increase temp = increase pH ( number actually gets smaller = increase acidity )

BUT remains neutral as still [H+] = [OH-]

Question 14

Why does increasing temp increase pH of water ( number gets smaller = increase acidity )

Answer 14

Increase temp
= equilibrium shift right ( forward endo )

= [H+] and [OH-] increases
= Kw increase
= pH increase

( but neutrality remains the same )

Question 15

Definition of neutral

Answer 15

[H+] = [OH-]

Question 16

In pure water what is true for the value ( or equation ) of Kw

Answer 16

Kw = [H+]^2

Because in pure water:
[H+] =[OH-]

Question 17

Kw increase with increasing temp.
When temp increases water

A. Remains neutral
B. Dissociates less
C. Becomes acidic
D. Becomes alkaline

Answer 17

A. Remains neutral

Question 18

How do u calculate the pH of stong base ( Method 1 ) ie. using [ OH - ]

Answer 18

1. [ H+ ] = Kw / [ OH-]

2. pH = -log[ H+ ]

Question 19

How do u calculate the pH of stong base ( Method 2 ) ie. using [ OH - ]

Answer 19

1. pOH = -log[ OH- ]

2. pH = 14 - pOH

Question 20

How do u calculate [ OH - ] in a strong base given the pH ( Method 1 )

Answer 20

1. [ H+ ] = 10^-pH

2. [ OH- ] = Kw / [ H+]

Question 21

How do u calculate [ OH - ] in a strong base given the pH ( Method 2 )

Answer 21

1. pOH = 14 - pH

2. [ OH- ] = 10^ -pOH

Question 22

What are the 5 steps for calculating pH of mixtures of strong acids and strong bases

Answer 22

1. Calculate moles of H+
2. Calculate moles of OH-

( moles = conc x vol( dm^3) )

3. Calculate moles of XS H+ OH-
   ( H+ XS = H+ - OH-)
   ( OH- XS = OH- - H+ )
4. Calculate conc of XS [ H+ ] or [ OH- ]
   ( conc = mol / ( total vol in dm^3 ) )
5. Calculate pH
   ( pH = -log[ H+] )
   ( pOH = - log[ OH- ] —> pH = 14-pOH )

Question 23

What does Ka stand for

Answer 23

Acid dissociation constant

Question 24

Give the the 3 formulas of Ka
( hint first one derived from equation of weak acid ( equilibrium equation) :

HA <=> H+ + A- )

Answer 24

1. Ka = [ H+ ] [ A ] / [ HA ]
2. Ka = [ H+ ]^2 / [ HA ]
   (only holds for weak acids in aqueous solution with nothing else added)
3. Ka = 10^-pKa

Question 25

Give formula for pKa ( a weak acid expression ) ( similar to formula of pH )

Answer 25

pKa = - log Ka

Question 26

What are the 2 assumptions for this this Ka formula:

Ka = [ H+ ] [ A ] / [ HA ]

Answer 26

1. [ H+ ] = [ A- ]
   Therefore Ka = [ H+ ]^2 / [ HA ]
2. [ HA ] is what’s given in the question
   ( so little has dissociated = we assume concentration has remained the same )

Question 27

What does a bigger Ka value tell you about an acid

Answer 27

Bigger Ka value = stronger acid

Question 28

What does a smaller pKa value tell you about an acid

Answer 28

Smaller pKa value = stronger acid