Required practical 1 Make up a volumetric solution and carry out a simple…

The idea

The practical has two halves. Making the standard solution: weigh the solid by difference, dissolve it in a beaker in less water than the final volume, then transfer the solution and the beaker washings into a volumetric flask, make up to the graduation mark with distilled water so the bottom of the meniscus sits on the line at eye level, stopper and invert repeatedly to mix. The washings matter because solute left in the beaker would lower the concentration. Titrating: rinse the burette with the solution it will hold, pipette a fixed volume into a conical flask, add a few drops of indicator, and run in until the first permanent colour change. Repeat until titres agree within 0.10 cm³ and average only those.

What earns the marks

• Transferring the solution together with the beaker washings into a volumetric flask.
• Making the solution up to the graduation mark with distilled water.
• Inverting or shaking the stoppered flask to mix thoroughly.
• Averaging only concordant titres, those within 0.10 cm³ of each other.
• Selecting an indicator with a sharp colour change over the vertical section of the titration curve.

Common mistakes and how to put them right

• Rinsing the conical flask with the solution being titrated, which adds extra moles and raises the titre.

• Including the rough titre when calculating the mean.

• Using an indicator for a weak acid against a weak base, where the pH change is too gradual to judge.

• Overshooting the graduation mark and then removing liquid with a pipette.

• Reading the top of the meniscus rather than the bottom.

Exam tips

• Quote burette readings to two decimal places ending in 0 or 5, and subtract to give the titre.
• Rinse the burette with the titrant and the pipette with the solution it will deliver, but the conical flask with distilled water only.
• When asked to improve accuracy, name a specific step such as washing the beaker into the flask.