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Chemistry Lab Exam > Titration of Acid using a PH meter > Flashcards

Titration of Acid using a PH meter Flashcards

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1
Q

what is a pH meter

A

an instrument used to measure the pH of a solution

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2
Q

what provides a direct method of obtaining a titration curve

A

pH meter

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3
Q

what is a titration curve

A

a graph of measured pH values verses the volume (mm) of the titrant added

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4
Q

how is the volume of one reagent needed to EXACTLY react with the other reagent measured

A

using a burette

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5
Q

what is a titration

A

a technique where a solution of known concentration is used to determine the concentration of an unknown solution

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6
Q

what is the analyte

A

the substance that is being analyzed (what was in the flask)

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7
Q

what is the titrant

A

the substance that is being used to analyze the analyte (what is run through the burette)

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8
Q

what was the titrant of this experiment

A

NaOH

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9
Q

what was the analyte of this experiment

A

the unknown acid

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10
Q

what is the equivalence point

A

the point where an equal amount (number of moles) of base has been added to the amount of acid present

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11
Q

where can the equivalence point be found on a titration curve

A

there is a relatively LARGE change in pH with a relatively SMALL change in volume

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12
Q

what is a monoprotic acid

A

acids that only have 1 acidic H to give up to the NaOH

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13
Q

what is the generalized formula for monoprotic acids

A

HA

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14
Q

how many equivalence points does a monoprotic acid have

A

1

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15
Q

how many equivalence points does a polyprotic acid have

A

more than 1

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16
Q

what is a polyprotic acid

A

acids that have more than 1 acidic H to give up to the NaOH

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17
Q

what is the generic formula for a diprotic acid

A

H2A

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18
Q

what is a diprotic acid

A

an acid that has 2 acidic Hs to give up to the NaOH

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19
Q

how many equivalence points does a diprotic acid

A

2

20
Q

what is the generic formula for a triprotic acid

A

H3A

21
Q

what is a triprotic acid

A

an acid that has 3 acidic Hs to give up to the NaOH

22
Q

how many equivalence points does a triprotic acid have

A

3

23
Q

what determines how precise the equivalence point has been determined

A

based on how steep the curve in the region is

  • those with a steeper curve have been more precisely established
24
Q

what is an endpoint

A

the point during a titration where the indicator changes colour

25
Q

how can the equivalence point of our titration curve be determined

A

a) use the steepest tangent line to the smooth curve where the pH changes rapidly

b) the equivalence point will be the midpoint between the two lines intersecting the VOLUME axis

26
Q

what is a disadvantage of the titration curve method

A

the time and effort required to make measurements and to construct the graph

27
Q

what is the equation for determining pH of a solution

A
28
Q

what is the equilibrium equation for a monoprotic acid in water AND how can the Ka value be found

A
29
Q

what does Ka stand for

A

ionization constant

30
Q

how can you determine the PKa of an acid from the titration curve

A

the Pka is equal to the Ph and can be read right off the graph for that acid

31
Q

how much of the unknown solid sample was obtained to start this experiment

A

1.25 to 1.75 grams

32
Q

how was the unknown acidic analyte solution prepared for this lab

A

1) first between 1.25 and 1.75 grams of the unknow solid sample was weighed using the technique weighting by difference (using weighing paper)

2) the weighed sample was transferred to a 250 mL beaker

3) then the weight sample was dissolved in about 50 mL of RO water

33
Q

what was done with the supplied 1 M NaOH solution for the experiment

A

about 100 mL of the 1 M NaOH solution was transferred to a clean, dry 150 mL beaker

34
Q

what is a hazard of working with NaOH

A

it is a strong base and can cause burns if left on the skin

35
Q

how were we suppose to clean the burette prior to doing the experiment

A
  1. first clean the burette 2 or 3 times with RO water
    - filling up a chunk of it with RO water and swirling the burette to ensure the entire surface of the glass is covered before opening the stop cock and letting the RO water drain down
  2. once it has been drained with RO water, run NaOH through the beaker once or twice to coat the sides in the base before filling it up the the zero mark to begin the titration
36
Q

if the NaOH does not exactly line up with zero when you start what must be done

A

just subtract that initial starting value from all the NaOH readings to ensure you calculate the NaOH that was actually added to the acidic solution

37
Q

what must be noted when the pH is meter is set up along with the rest of the apparatus

A

it is clamped so that the electrode is immersed in the acidic analyte solution and off to one side of the beaker so that the burette will drip the NaOH solution into the solution but NOT on the pH meter electrode

38
Q

how should the drops of the NaOH change once there is a rapid change in pH of the solution

A

drops should be reduced to about 0.5 mL or less compared to the initial 0.1 mL

39
Q

when should you conclude adding the NaOH to the acid solution

A

once the equivalence point(s) have been reached OR if the pH is about 11 or 11.5

40
Q

how can the titrated solution be disposed of

A

down the sink with LOTS of tap water (after being diluted)

41
Q

what is done with the magnetic stir rod after the titration

A

MUST NOT go down the sink and give back to the TA

42
Q

what can be done with excess NaOH

A

diluted to 1 M and poured down the sink with lots of water

43
Q

generic equation of NaOH reacting with HA

A

NaOH (aq) + HA(aq) —–> H2O(l) + ANa(aq)

44
Q

generic equation of NaOH reacting with H2A

A

2NaOH (aq) + H2A(aq) —–> H2O(l) + ANa2(aq)

45
Q

generic equation of NaOH reacting with H3A

A

3NaOH (aq) + H3A(aq) —–> 3H2O(l) + ANa3(aq)

46
Q

how to determine Ka from Pka

A

Ka = 10^-pka

Chemistry Lab Exam (4 decks)

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