Chemistry Core Practical 2: Finding the concentration of a solution of sodium hydroxide

CP 3 is almost the same, so just use these to revise it x

Safety

Safety

• Wear eye protection (goggles)

• Sulfamic acid can be toxic if it is ingested

• Ensure burettes are filled when the top the burette is below eye level

Equipment

• Burette clamp and stand

• Solid sulfamic acid

• Sodium hydroxide solution of unknown concentration

• Methyl orange indicator

• 250 cm³ conical flask

• 25 cm³ volumetric pipette plus safety filler

• 100 cm³ beaker for transfer of solutions

• funnel for filling burette

• 250 cm³ beaker

• 250 cm³ volumetric flask

• Mass balance (2 d.p.)

• Eye protection (goggles)

Procedure

1. Weigh an empty test tube. Scoop approximately 2.5g of sulfamic acid into the test tube.

2. Right the test tube and its contents accurately.

3. Dissolve the sulfamic acid in approximately 100 cm³ of water in a beaker.

4. Transfer the solution, including the washings, into a 250 cm³ volumetric flask and make the solution up to the mark with deionised water.

5. Prepare your apparatus for the titration. The burette will contain the acid solution and the conical flask will contain the sodium hydroxide solution.

6. Pour 25.0 cm³ aliquot of sodium hydroxide unknown concentration into the 250 cm³ conical flask.

7. Add four drops of methyl orange indicator to the conical flask.

8. Titrate the contents of the flask against the sulfamic acid solution you have prepared. Burette readings should be recorded to the nearest 0.05 cm³. Continue to conduct titrations until you have two concordant titres.

9. Record you results in a table.

Apparatus set up

• Rinse burette with solution being measured in burette

• Rinse pipette with solution being measured in pipette

• Ensure jet below tap is filled (no air bubbles)

• Remove funnel before titration

• Place white tile beneath conical flask

• Dropwise addition near endpoint

4. Transfer the solution, including the washings, into a 250 cm³ volumetric flask and make the solution up to the mark with deionised water.

What are the washings in Step 4 and why are they important for an accurate standard solution?

• Small volumes of deionised water used to rinse out the beaker and glass rod.

• Washings contain some of the sulfamic acid, so must be added to the volumetric flask to ensure that the standard solution contains all the acid measured, and therefore is the concentration calculated using the flask volume.

Reaction equation

H₃NSO₃ (aq) + NaOH (aq) —> HaH₂NSO₃ (aq) + H₂O (l)