practical 1

Describe how to make a standard solution

1. Accurately weigh out the sample bottle containing the solid on a 2dp balance

2. Transfer solid to beaker and reweigh sample bottle

3. Record difference in mass

4. Add distilled water and stir with glass rod until all the solid has dissolved

5. Transfer to a volumetric flask with washings

6. Make up to the 250cm mark with distilled water

7. Shake flask/invert

Give 2 ways to reduce uncertainties in measuring mass

• Use a more accurate balance or a larger mass

• Weigh sample before and after addition and calculate difference in mass

Describe a method for determining the concentration of an unknown solution of HCl, using a 1.0 moldm-3 solution of sodium hydroxide by acid-base titration

1. Rinse burette with sodium hydroxide to remove any contamination

2. Overfill burette with sodium hydroxide and run through so jet space isn’t filled

3. Record initial burette volume

4. Use a volumetric pipette to transfer 25cm3 of HCl to conical flask

5. Add 2 drops of indicator (phenolphthalein)

6. Slowly add NaOH to the acid

7. Swirl between additions

8. Stop at permanent colour change

9. Record final burette reading

10. Calculate volume of NaOH added (titre volume)

11. Repeat titration until concordant data recorded

12. Use concordant data to calculate mean titre

13. Use n=c x v to calculate moles of NaOH added

14. As they react in a 1:1 ratio, use moles of NaOH to calculate concentration of HCl

What are concordant results?

Titres that are within 0.1cm3 of eachother

Why should the burette be rinsed out with the substance that will be put in it?

So the acid or alkali that is put into it wont be diluted by residual water, or react with substances left from a previous titration (contamination)

This would lower the concentration and therefore give a larger titre reading

Why shouldn’t you leave the funnel in the burette?

Small drops of liquid may fall from the funnel into the burette during the titration, which would give a lower titre volume

Why should the jet space in the burette be filled?

If not, it will lead to errors if it then fills during the titration, giving a larger than expected titre reading

Explain when phenolphthalein should be used, its pH range and give the colour change

• Use with titrations using strong alkalis eg NaOH as it changes colour between pH 8.2 and 10.0

• Pink in alkali, colourless in acid

Explain when methyl orange should be used, its pH range and give the colour change

• When strong acids are used eg HCl, as its pH range is 3.1 to 4.4

• Red in acid (orange at end point), yellow in alkali

How can you reduce the uncertainty in a burette reading?

Make the titre a large volume by increasing volume and concentration of substance in conical flask or decreasing the concentration of the substance in the burette