RP: Titrations Flashcards
Why do we do “back titrations?”
- Used to work out information about an **unreactive **base.
Why do we use a burette for 1 solution?
- Delivers an accurate volume (that can be read.)
- Measures the variable volume.
Why do we place one solution in a conical flask (in normal titration?)
- Allows for swirling without spillages.
Why do we use back titrations rather than normal titrations?
- Back titrations are used when the base is quite unreactive.
What are the steps to a back-titration ie. calculating the moles/amount of an unreactive base?
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.
Why do react the unreactive base with EXCESS acid in back titrations?
- This ensures ALL of the base is used up.
What is the “titre?”
- Subtraction of final burette reading from intial burette reading.
Two indicators to be used in typical acid- base titrations. Colours they are in acid/ alkali?
1.) Methyl orange (yellow in alkalis, red in acids.)
2.)Phenolphthalein (colourless in acids, pink in alkalis.)
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?)
- Uncertainty/ reading x 100
- More than one reading? Need to multiply uncertainty by number of times you took reading/ reading taken x 100
How do we reduce percentage error in titrations?
2 main ways
1.) Take the largest reading possible ie. larger volume/ larger mass.
2.) Use equipment with higher resolution (less uncertainty.)
What do we use to measure 25cm3 of known solution into conical flask?
- Use a pipette.
Method for making standard solution of sodium hydrogensulphate between 0.09 and 0.11 moldm-3
8 main steps
1.)Measure mass of weighing boat and solid.
2.) Add the solid to a beaker and then reweigh the weighing boat, subtract from mass of weighing boat + solid to find mass of solid added.
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.
When you weigh anything on the 2dp balance, to how many decimal places should you record all masses in table?
- 2 decimal places.
Why do we measure the mass of the weighing bottle and its contents after we have transferred the solid?
- Not all required substance will have been transferred.
What could be an issue with dissolving the sodium hydrogensulphate solid?
- May not dissolve in cold water (so beaker/ content could be heated until all of solid dissolves.)
What pieces of equipment do we wash into the volumertric flask and why?
1.) Glass rod
2.) Funnel
3.) Beaker.
Ensures that solution lost during transfer is kept to a minimum.
How can you ensure that you don’t overshoot when filling the volumetric flask? Why is it important not to overshoot?
- You could use a dropping pipette.
- Important not to overshoot so we have a **known **volume.
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?
- 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
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?
- 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.
How can we reduce uncertainty in measuring mass?
- Use balance with higher resolution/ weigh a larger mass.
Method for titration
9 steps
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.
What type of beaker should we placing the NaOH and NaHSO4 in?
- Use a clean, dry beaker.
What do we do before filling the burette with the SD solution (in this case, sodium hydrogensulphate?)
2 main things
- 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.)
What do we do before filling up pipette with 25cm3 of solution of unknown conc (in this case NaOH)?
- Rinse the pipette with solution of unknown concentration (ie. NaOH)
