Required Practical 1 - Titration

Describe how to make a volumetric solution for an acid-base titration.

1. Weigh the desired mass of solid required

2. Add 100cm³ of distilled water to the beaker and use a glass rod to dissolve the solid (may need to heat gently to dissolve)

3. Transfer to a 250cm³ volumetric flask 

4. Rinse beaker with distilled water and add contents to the volumetric flask 

5. Fill the flask up to 250cm³

6. Invert several times to make a uniform solution 

Describe how to weigh out the solid for a volumetric solution.

1. Weigh the bottle containing the solid on a balance

2. Transfer to a beaker

3. Reweigh the empty bottle

4. Subtract the mass of the bottle from the total mass 

How can you ensure the flask is filled exactly 250cm³?

Make sure the meniscus sits on the line 

Why should hot solutions not be put in a volumetric flask?

It can cause the flask to expand and the volume to be incorrect

Why is using a volumetric pipette more accurate than a measuring cylinder?

It has a smaller uncertainty

Describe how to carry out a simple acid-base titration.

1. Rinse equipment with distilled water 

2. Pipette 25cm³ alkali into a conical flask 

3. Add a few drops of methyl orange to the flask 

4. Fill the burette with acid and note the original volume 

5. Slowly add the acid to the conical flask from the burette, swirling constantly

6. Add dropwise when the solution starts to change colour 

7. Stop adding when the mixture changes colour completely and note the final reading on the burette

8. Repeat to find concordant results and calculate the mean volume of acid used 

Why is a conical flask used instead of a beaker?

It is easier to swirl the mixture without spilling the contents

How do you clean the burette before using it for the experiment and why?

Rinse it out with the solution that will be in it to ensure there are no impurities when the experiment is carried out (this would make the titre appear lower)

Why should you make sure the jet space is filled with the solution before starting the titration? 

So that the titre value is accurate (not filling it leads to a larger titre than expected)

When should phenolphthalein be used as an indicator?

When using strong alkalis like NaOH

What colour change when phenolphthalein is used as an indicator?

Pink to colourless (as soon as it disappears)

Why should only a few drops of indicator be added?

Too much can affect the titration result as they tend to be weak acids

When should methyl orange be used for a titration?

When using strong acids

What is the colour change when methyl orange is used as an indicator? 

Yellow to orange (red in acid) 

Why should distilled water be added to the conical flask during a titration?

To wash the sides of the flask so that all of the acid is washed into the reaction mixture to react

Why does adding distilled water to the flask not effect the titration reading?

It does not react or change the moles of acid that are present

What are concordant results?

Results that are within 0.10cm³ of each other

State the safety precautions when carrying out required practical 1. 

Acids and alkalis are corrosive (acids are irritants at low conc.) so wear gloves

Wear eye protection at all times