titrations prac - errors

Question 1

what is a systematic error

Answer 1

error is always above or below the correct value

Question 2

what is a random error

Answer 2

error that is unpredictably above or below correct value

Question 3

give examples of systematic errors

Answer 3

incorrect indicator

judging colour change at end point

rinsing burette with water

rinsing pipette with water

unclean equipment

Question 4

give examples of random errors

Answer 4

parallax error

precision of burette

precision of pipette

precision of balance

Question 5

explain the error of choosing the wrong indicator

Answer 5

end point occurs before/after equivalence point

titre volume will always be less/greater than correct value

affects calculated concentration

Question 6

explain the error of judging the colour change at the end point

Answer 6

overshoot resulting in more intense indicator colour will result in higher titre

calculated concentration will be lower

Question 7

explain the error of prerinsing the burette with water

Answer 7

remaining water in burette will dilute solution

give a greater titre volume

calculated concentration will be lower

Question 8

explain the error of prerinsing the pipette with water

Answer 8

remaining water in pipette will dilute solution

lower titre volumes

calculated concentration is higher

Question 9

explain the error of using unclean equipment

Answer 9

lower volume of solution delivered

affects calculated concentration

Question 10

explain what a parallax error is

Answer 10

misreading the bottom of the meniscus

randomly results in errors above or below correct value

Question 11

explain the the precision of the burette can be an error

Answer 11

e.g. if the smallest scale division in a burette is 0.1mL, the titre volume may be 0.05mL above or below the correct value

Question 12

explain the the precision of the pipette can be an error

Answer 12

volume of solution delivered may be slightly above or below correct value

Question 13

explain the the precision of the balance can be an error

Answer 13

balances usually round to the half least scale division

any result is unpredictably above or below half the least scale division

Question 14

e.g. You are titrating an acid into a base. The endpoint is reached but a few more drops fall out of the burette into the base. How does this affect the base concentration calculations?

Answer 14

1. extra acid drops increased number of moles of acid required to neutralise base
2. this increases number of moles of base
3. this increases concentration of base in calculations

Question 15

e.g. Drops of water are left in a burette, which then titrates a base into an acid. What is the effect of water in the burette on the acid concentration?

Answer 15

1. drops of water decrease concentration of base
2. therefore increasing volume of base needed to neutralise acid
3. this increases number of moles of acid
4. therefore increases conc. of acid
5. concentration of acid is inflated