acids, bases, ph

Question 1

explain which indicator is most suitable for the titration

Answer 1

name*- ph range matches vertical section/equivalence point

Question 2

explain why the measured ph is different from calculated ph

Answer 2

named reactant* dissociation is not negligible
large Ka and is ‘stronger’ weak acid
eq. conc. is significantly lower than initial conc

Question 3

details to add to a qs on how buffer controls ph

Answer 3

write equilibrium equation
state addition of H+ cause shift in eq to left
addition of OH- cause shift in eq to (right)
increase in H+ leads to: eg write equation-formation of HA
increase in OH- leads to: eg water forms so decreases H+
in this way when small amounts of acid or alkali are added the conc of H+ ions remain constant and therefore pH remains constant

Question 4

relationship between H+ and A- in weak acid

Answer 4

[H+]= [A-]

Question 5

what factors determine the ph of buffer solution

Answer 5

temperature
ratio/conc. of weak acid and conjugate base
Ka of solution

Question 6

what is a conjugate acid-base pair

Answer 6

2 species which differ by H+

Question 7

when does a buffer work best

Answer 7

[HA]=[A-]

Question 8

when using Ka eq. for weak acids, what are the assumptions made

Answer 8

1. very little HA dissociates
2. [H]=[A] => Ka=[H]^2/[HA]

Question 9

describe how the student can recrystallise impure crystals to obtain pure crystals

Answer 9

1. dissolve crystals in MINIMUM quantity of HOT water/solvent
2. cool AND fiter (usually under reduced pressure) AND leave to dry

Question 10

explain whether dissociation of water is endo or exothermic process

Answer 10

endo- dissociation (Kw) increases with temp (involves breaking bonds)