Ph and indicators

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Base dissociation constant

(B+)(OH-)/(BOH)

Ph titration

1. Base in burette acid in beaker, start with low ph 2. Stir with ph sensor(measures h+ ion conc) 4. Graph ph vs time using data logger 5. No indicator

Limitations

1. Limited to ph in 0-14 range 2. Doesn't work with v conc solns as complete dissociation doesn't always occur(can't predict) 3. Limited to aqueous solutions

Why is ph always 7

1. Same number of h+ as Oh- ions as for every h+ formed when a h20 dissociates an oh- is also formed

Ionic product of water

1by10tothe-7

The self-ionisation of water

2H2O -> h3O+ + OH-

Strong acid strong base

3 to 10 Large jump cause by addition of one drop- sharp Any indicator that's one colour at 3 and another at 10

Methyl orange

3 to 5 Acid- red Base- yellow

Strong acid weak base

3 to 7 Methyl orange- range 3 to 7 -litmus doesn't change till 8 -phenol range above

Litmus

5 to 8 Acid- red Base- blue

Weak acid strong base

7 to 10 Phenol- range lies within vertical part

Phenolphthalein

8 to 10 Acid- colourless Base- pink

Strong acid

A good proton donor eg hcl h2so4 hno3 - almost completely dissociates

Universal indicator

A mixture of many

A weak acid

A poor proton donor only slightly dissociates

Acid base indicator

A substance that changes colour according to the ph of the solution in which it is placed

The ph scale

Acidic solution-less than 7 Neutral solution-equal to 7 Alkaline soln- greater than 7

Calculating ph strong acid strong base

Assumed to have completely dissociated in water (see notes)

When they're a weak acid

Colour of dissociated molecule is different to that of the dissociated molecule

Structure of indicators

Contain lots of benzene rings

Ph of weak acids and weak bases

Depends on value of dissociation constant and solutions concentration (see notes)

Why different ranges

Each indicator has their own dissociation constant

When h+ ions added

Eq to left to absorb (acidic)

When oh- ions added

Eq to right as h+ combines with oh- to make water so more h+ must be made

Strong base

Good proton acceptor eg. Naoh and koh

Acidic solution

H+ greater than oh-

Basic solution

H+ less than Oh-

Strong acid

Has a weak conjugate base

Acid dissociation constant

Ka = (H+)(A)/(HA) (H20 doesn't appear but use if asked for kc)

Kw

Kc(h2o) = (H+)(OH-)

Weak base indicators acid added

Oh- and h+ form water so shifts to right

Weak acid weak base

Ph rises gradually so not possible to detect end point using indicator as no sudden jump- not used to find concs

Graph name

Ph titration curve

Weak base

Poor proton acceptor

Base added

Shifts to left to absorb excess oh-

Weak acid

Strong conjugate base

Dependent on

Temp- increases with increasing temp as ph falls

Values of Ka

The larger the value the higher is the concentration of h3o+ and the stronger the acid

The ph of a solution

The negative logarithm to the base 10 of the hydrogen ion conventration measured in moles per litre

Range of an indicator

The ph interval over which there is a clear change of colour for that indicator

Ie

The stronger the acid the weaker the conjugate base The weaker the acid the stronger the conjugate base

Measuring ph

Universal indicator Ph sensor and data logger sensitive to h+ ion conc eg. Hospitals

Meaning

Variation in h2o v small in comparison to its size The ionic product of water

Ion concentration

Very low so equilibrium lies much to the left


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