RP: Titrations

Question 1

Why do we do “back titrations?”

Answer 1

• Used to work out information about an **unreactive **base.

Question 2

Why do we use a burette for 1 solution?

Answer 2

• Delivers an accurate volume (that can be read.)
• Measures the variable volume.

Question 3

Why do we place one solution in a conical flask (in normal titration?)

Answer 3

• Allows for swirling without spillages.

Question 4

Why do we use back titrations rather than normal titrations?

Answer 4

• Back titrations are used when the base is quite unreactive.

Question 5

What are the steps to a back-titration ie. calculating the moles/amount of an unreactive base?

Answer 5

1.) Add (unreactive) base in excess acid.
2.) Titrate the left over/ excess acid with a base (ie. sodium hydroxide)
3.) Calculate moles of excess acid (as you normally would in titration Q.)
4.) Calculate moles of acid used up in reaction by doing start moles (in the excess) -left over moles (from the titration.)
5.) Hence, work out moles of unreactive base.

Question 6

Why do react the unreactive base with EXCESS acid in back titrations?

Answer 6

• This ensures ALL of the base is used up.

Question 7

What is the “titre?”

Answer 7

• Subtraction of final burette reading from intial burette reading.

Question 8

Two indicators to be used in typical acid- base titrations. Colours they are in acid/ alkali?

Answer 8

1.) Methyl orange (yellow in alkalis, red in acids.)
2.)Phenolphthalein (colourless in acids, pink in alkalis.)

Question 9

How do you calculate percentage error when you have taken ONE READING FROM THE EQUIPMENT? How about if you have taken more than one reading from the equipment (ie. burette - intial and final reading got overall reading?)

Answer 9

• Uncertainty/ reading x 100
• More than one reading? Need to multiply uncertainty by number of times you took reading/ reading taken x 100

Question 10

How do we reduce percentage error in titrations?

2 main ways

Answer 10

1.) Take the largest reading possible ie. larger volume/ larger mass.
2.) Use equipment with higher resolution (less uncertainty.)

Question 11

What do we use to measure 25cm3 of known solution into conical flask?

Answer 11

• Use a pipette.

Question 12

Method for making standard solution of sodium hydrogensulphate between 0.09 and 0.11 moldm-3

8 main steps

Answer 12

1.) Place weighing bottle on balance (zero balance.)
2.) Weigh the weighing bottle (containing the solid) on a (2dp) balance.
3.) Transfer the solid to the beaker and reweigh the weighing bottle.
4.) Record difference in mass (this is the mass of sodium hydrogensulphate transferred.)
5.) Add distilled water to the sodium hydrogensulphate in beaker/ stir with glass rod until all solid has dissolved.
6.) Use a funnel to pour contents of beaker into 250cm3 volumetric flask. Using distilled water, wash out the funnel/ beaker (after you have poured the contents), transfer all washings to volumetric flask.
7.) Make volumetric flask up to 250cm3 by adding distilled water.
8.) Stopper the volumetric flask/ and shake.

Question 13

When you weigh anything on the 2dp balance, to how many decimal places should you record all masses in table?

Answer 13

• 2 decimal places.

Question 14

Why do we measure the mass of the weighing bottle and its contents after we have transferred the solid?

Answer 14

• Not all required substance will have been transferred.

Question 15

What could be an issue with dissolving the sodium hydrogensulphate solid?

Answer 15

• May not dissolve in cold water (so beaker/ content could be heated until all of solid dissolves.)

Question 16

What pieces of equipment do we wash into the volumertric flask and why?

Answer 16

1.) Glass rod
2.) Funnel
3.) Beaker.
Ensures that solution lost during transfer is kept to a minimum.

Question 17

How can you ensure that you don’t overshoot when filling the volumetric flask? Why is it important not to overshoot?

Answer 17

• You could use a dropping pipette.
• Important not to overshoot so we have a **known **volume.

Question 18

What is the issue with balance giving value of 0.01g when it says its at 0g? What could we do to ressolve this issue?

Answer 18

• Causes systematic error.
• Transfer solid in weighing bottle to beaker.
• Wash all remaining solid from sample into beaker.
  (rather than doing subtraction of potentially inaccurate values - would increase in- accuracy even more. )
  () - extra info for clarification

Question 19

With what kind of solutions will it be difficult to see the meniscus when making volumetric flask up to 250cm3? What could we do to ressolve this?

Answer 19

• Dark liquids = may be difficult to see meniscus.
• Place white piece of paper (behind the volumetric flask) to make the marking point clearer to see.

Question 20

How can we reduce uncertainty in measuring mass?

Answer 20

• Use balance with higher resolution/ weigh a larger mass.

Question 21

Method for titration

9 steps

Answer 21

1.) Pour SD solution into a beaker.
2.) Fill burette with SD solution of known concentration (using funnel.)
3.) Pour solution with unknown concentration into another beaker.
4.) Use pipette filler/ pipette to transfer 25cm3 of solution into conical flask.
5.) Add two to three drops of phenolphthalein indicator to the solution in the conical flask/ record colour of indicator.
6.) Record initial burette reading.
7.) Titrate contents of conical flask by adding SD solution (from burette) until indicator undergoes clear colour change.
8.) Record the final burette reading. Calculate the titre.
9.) Repeat until two concordant results are obtained.

Question 22

What type of beaker should we placing the NaOH and NaHSO4 in?

Answer 22

• Use a clean, dry beaker.

Question 23

What do we do before filling the burette with the SD solution (in this case, sodium hydrogensulphate?)

2 main things

Answer 23

• Rinse the burette WITH SD solution.
• Make sure jet space is filled/ doesn’t contain air bubbles (if not filled this will lead to errors when it fills during titration - larger titre reading.)

Question 24

What do we do before filling up pipette with 25cm3 of solution of unknown conc (in this case NaOH)?

Answer 24

• Rinse the pipette with solution of unknown concentration (ie. NaOH)

Question 25

What should we do to the conical flask before transferring 25cm3 of solution of unknown conc to it?

Answer 25

• Should be rinsed with distilled water.

Question 26

Why is important that we only add two/ three drops of phenolphthalein to solution in conical flask?

Answer 26

• Too much added- this will affect titration result.

Question 27

How should we titrate the contents in the conical flask?

3 main points.

Answer 27

• Add solution (from burette) slowly, swirling flask to mix the solution.
• Add solution dropwise near the end-point.
• Use a white tile underneath the conical flask to make the colour change clear.

Question 28

What can you do to the sides of a conical flask during a titration? Why is it that this step doesn’t have an effect on the titration?

Answer 28

• Distilled water can be used to wash sides of flask so all reactants are washed into mixture.
• No effect: it doesn’t change the number of moles of each reactant (ie. you still have 25cm3 of NaOH.)

Question 29

True or False

You should carry out at least 2 titrations in this practical.

Answer 29

• False.
• You should carry out at least 3 titrations.

Question 30

If solution A is titrated against solution B, what does this mean about which one is in conical flask/ in burette?

Answer 30

• Solution A in conical flask.
• Solution B in burette.

Question 31

To how many dps should you record volumes in this practical?

Answer 31

• 2 dps.

Question 32

What is the main risk in this pratical and what is a precaution that can be taken for this risk?

Answer 32

• Acids/ alkalis are corrosives/ irritants SO –> wear goggles/ gloves.

Question 33

Why might you titrate a mixture? What do you have to consider when titrating a mixture?

Answer 33

• Work out concentration of active ingredient (that is either an acid/ base.)
• Important to consider if mixture contains other substances that have acid- base properties.

Question 34

When testing batches (big volumes) of a solution, why is it important to test on several samples?

Answer 34

• As concentration/ amount of chemical being tested may vary between samples.

Question 35

How would you calculate the “absaloute” uncertainty (in a titration) of a burette with uncertainty of +/– 0.05 cm3? What is often added onto this uncertainty?

Answer 35

+/– 0.05 cm3 x 2 (because two readings are taken) = +/– 0.10 cm3.
- Often, another 0.05 cm3
is added on because of uncertainty identifying the end point colour change.

Question 36

3 ways you can reduce uncertainty in a titration.

Answer 36

1.) Replace measuring cylinders with pipettes (have lower uncertainty.)
2.) Reduce uncertainty in burette reading? –> titre volume needs to be larger.
3.) Leaving substances in burette will cause damage to apparatus (could lead to errors.)

Question 37

How can we make titre volume from burette larger/ reduce the uncertainty from the burette?

2 main ways.

Answer 37

• Increasing volume/ concentration of substance in conical flask.
• Decreasing concentration of substance in burette.

Question 38

Where should solution with unknown concentration be? In burette/ in concial flask?

Answer 38

• In conical flask.

Question 39

What are the chemical formulas for the chemicals we use in the RP titiration? Which one is known? Which is unknown? Give balanced equation for reaction.

Answer 39

• NaHSO₄ (known concentration/ SD solution.)
• NaOH (unknown concentration.)
• NaHSO₄ + NaOH –> NaSO₄ + H₂O

Question 40

In this RP, which substance tends to be in conical flask/ which tends to be in the burette?

Answer 40

• Conical flask: sodium hydroxide.
• Burette: sodium hydrogensulphate.