Required Practical 1: Make up a volumetric solution and carry out a simple acid - base titration Flashcards
What is a titration?
Titrations are a form of volumetric analysis, used to find the unknown chemical value
What is a standard solution?
A standard solution is a solution of known concentration
How do you make a standard solution?
1) Weigh the weighing boat bottle on a 2 dp balance, recording mass
2) Then reweigh the weighing boat containing the required mass of the sample on a 2 dp balance
3) Record the difference in mass and transfer to a beaker
4) Add 100cm3 of distilled water to the beaker. Use a glass rod to
stir to help dissolve the solid.
5) Pour solution into a 250cm3 graduated flask via a funnel.
6) Rinse beaker and funnel and add washings from the beaker
and glass rod to the volumetric flask.
7) Make up to the mark with distilled water using a dropping
pipette for last few drops.
8) Invert flask several times to ensure uniform solution.
Sometimes the substance may not dissolve well in the cold water. In this case, what do you do to ensure all the substance has dissolved?
You would gently heat the beaker until all the substance has dissolved
How do you reduce the uncertainties when measuring the masses?
1/ Use a balance which has a higher resolution
2/ Measure a larger mass
3/Weigh weighing boat before and after addition of the solid sample
What equipment is used to carry out a titration?
A pipette and pipette filler are used to accurately measure out the volume of a reactant before transferring it to a conical flask
A burette is used to add small volumes of one reactant to the other reactant (until the reaction has reached completion)
How do you carry out a titration?
1) Once the pipette has been used to place one reactant into the conical flask, fill the burette with other reactant. Record initial volume
2) Add a few drops of indicator to the conical flask
3) Open the burette tap and allow the reactant to flow into the conical flask, swirling it to mix the contents
4) Close the burette tap once the expected colour change occurs, Use a white tile so the colour change is easy to identify
5) Record final burette volume
6) Repeat until you get concordant results, then calculate a mean titre
Why is it okay to use distilled water to wash the sides of the conical flask?
The water does not affect the titration as it does not change the number of moles of each reactant
Why must jet space in burette be filled, ensuring it doesn’t contain air bubbles?
If jet space is not filled it will lead to errors if it then fills during the titration, it will lead to a larger than expected titre reading
Why are acid - base indicators used?
To detect when a reaction reaches completion, usually by the presence of a colour change
What are concordant results?
Titres that are within 0.1cm cubed of each other
What are the safety precautions used in this required practical?
Wear eye protection and gloves
The student uses a funnel to fill the burette with sulfuric acid before starting the titration. After filling, the student forgets to remove the funnel from the top of the burette
Suggest why this might affect the titre volume recorded
Solution may drip from the funnel so volume measured in the burette would end up being lower
Why does a conical flask work best for titrations?
The shape of the conical flask works best as it allows for more vigorous swirling and minimises spillages more than other glassware
Why should only a few drops of indicator be used?
This is because the indicator is a weak acid itself, so by adding too much of it may skew the acidity leading to inaccurate results
What are different ways to increase the accuracy of the experiment?
1/ Carry out min. three titrations
2/ Make sure all burette readings are taken from eye level at the bottom of the meniscus
3/ Place white tile at bottom of conical flask to ensure
Why are multiple titrations carried out, why is just one not sufficient?
A single titration could be flawed, so repeating titres allows for anomalous titres to be spotted and discounted
The student rinses the burette with deionised water before filling with sodium hydroxide solution.
State and explain the effect, if any, that this rinsing will have on the value of the titre
The titre values will become skewed because the water will dilute the sodium hydroxide causing larger titre values and a lower concentration of sodium hydroxide
What is the equation to calculate mean titre result?
Mean Titre = concordant results divided by number of concordant results
How is percentage uncertainty calculated?
%uncertainty = (+- uncertainty divided by measurement) x 100
Why would you not heat a volumetric flask?
You should not heat or put hot solutions in the volumetric flask because the heat would cause the flask to expand and the volume would then be incorrect
What is the percentage uncertainty of the following:
A. Burette
B. Pipette
C. Volumetric flask
D. Balance
A. Burette = +_ 0.05 (x2)
B. Pipette = +_ 0.05
C. Volumetric flask = +_ 0.05
D. Balance = +_ 0.01 (x2)
