module 2 - acid-base titrations Flashcards
what’s a titration?
a technique used to accurately measure the volume of one solution that reacts exactly with another solution
what are 3 things can titrations be used for?
- finding concentration of a solution
- identification of unknown chemicals
- finding the purity of a substance
what’s a standard solution?
a solution of known concentration
what is used to make a standard solution?
a volumetric flask
what is a volumetric flask used for in titrations?
to make up a standard solution very accurately
why is using a volumetric flask good?
because they can measure volumes very precisely
what are the 1st 3 steps in preparing a standard solution?
- weigh solid accurately
- dissolve solid in a beaker using distilled water (less than what will be needed to fill the vol flask to the mark)
- transfer this solution to vol flask. (last traces of solution are rinsed into the vol flask with distilled water)
what next step is taken after rinsing and solution are first put into the volumetric flask?
- flask is filled to the graduation line by adding distilled water one drop at a time until the bottom of the meniscus lines up exactly with the mark
why is it important to be careful when filling up the volumetric flask completely?
- if too much water is added, the solution = too dilute and must be made again
how should you view the graduation mark and meniscus and why?
- at eye-level
- for accuracy
what’s the final step after the volumetric flask is completely filled?
- its slowly inverted several times, to mix the solution thoroughly and evenly
what happens if you dont invert the mixture?
- titration results are unlikely to be consistent
how can you see the solution mixing when yo invert the vol flask?
the more dense original solution moves through the solution
what is the effect on the titre with the error of filling the flask with water above the grad line?
the titre would be less as solution is more dilute
what is the effect on the titre when in error, the flask is not inverted?
- the solution used in the first titration would be more dilute than the solution used after.
- The first titre would need less solution
in an acid-base titration what is titrated against what?
a solution of an acid is titrated against a solution of a base using a pipette and a burette
how is a burette reading recorded?
to the nearest half division
what is each burette reading measured to? what does this mean?
to the nearest +/- 0.05cm^3
- the reading always has 2 dp
- last decimal place is always 0/5
what should you do when filling a burette to remove air bubbles?
run excess solution out through the tap
where would you put the acid and alkali in a titration to find the concentration of hydrogen ions in an acid solution
- put acid in burette
- put alkali in conical flask
what are the 1st 3 steps in doing an acid-base titration (before turning tap on)??
- add a measured volume of 1 solution (usually unknown one) to a conical flask using a pipette
- add the other solution (usually the standard solution) to the burette, and record the initial burette reading to nearest 0.05cm^3
- add a few drops of indicator to the conical flask solution!!
for a titration, what is the step after the indicator is put in?
- run burette solution into conical flask, swirling the con. flask throughout it to mix both.
- soon, the indicator changes colour at the end point of the titration
what does the end point do?
it’s used to indicate the volume of 1 solution that exactly reacts with the volume of the 2nd solution.
after the end point is reached in a titration what is the next step?
- record the final burette reading
what is the titre? How is it found?
- The volume of solution added from the burette
- By subtracting the initial from the final burette reading! (F-I)
when doing titrations, what is always done first? why?
- a quick trial titration
- to find the approximate titre
how many times are titrations accurately repeated?
until 2 accurate titres are concordant (they’re within 0.10cm^3 of each other)
{e.g. between 24.60, 24.30 and 24.80, if concordant criteria was 0.20, then only 1 and 3 are concordant. 2 is irrelevant}
in titrations, how should the burette solution be put in towards the endpoint?
dropwise until endpoint reached
what is the tire also known as?
the volume of the solution that’s been added
which titration result is ignored in calculations?
the rough titre
when calculating mean titre, what do you include and what dont you?
- include the titres that are concordant
- don’t include any that are too low or too high
what should u have in your titration results table?
- final burette reading in cm^3
- initial burette reading in cm^3
- titre in cm^3
- mean titre in same units
- boxes for trial and for 3 repeats
explain the effect of the error of the pipette having an air bubble inside?!
tire would be less as less solution has been added than the correct pipette volume
explain the effect of the error of the burette readings being taken from the top of the meniscus?
no effect as volume is measured by difference between two readings.
explain the effect of the error of the burette having an air bubble inside?!
if a bubble is left in the neck of the burette, the air could be released during the titration leading to an error in the titre
when working out mean titre, what is it important to use?
- only the closest accurate titres
what would happen if you included all the titres in the mean?
you lose the accuracy of the titration technique
what 2 things will you know from the results of a titration?
- the concentration and reacting volume of one of the solutions
- just the reacting volume of the other solution
what is the pattern (3 steps) used to find out info about the unknown compound?
1) work out the amount in mol of the solute in the solution (the one that u know the conc and vol for)
2) use equation to find amount in mol of the solute in the other solution
3) work out the unknown info about the solute in the other solution
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