4.1.1 Required Practical 1 - Making a Volumetric Solution

Aim

To prepare a volumetric solution of a known concentration by accurately dissolving a solid in distilled water, and to make up to a specified volume using a volumetric flask.

Equipment

Chemicals

• Solid to be dissolved (e.g., sodium hydrogensulfate, $NaHSO₄$)
• Distilled water

Apparatus

• 2 decimal place (2 dp) balance
• Weighing bottle or sample bottle
• 250 cm³ beaker
• Glass rod for stirring
• 250 cm³ volumetric flask with stopper
• Funnel for transfer
• Dropper or pipette for adjusting the final volume

Risk Assessment

• Sodium hydrogensulfate can cause irritation to eyes and skin:
  • Wear safety goggles and lab coat.
  • If contact occurs, rinse immediately with water.
• Handle glass equipment with care to avoid breakage.
• Use a fume cupboard if preparing a volatile or hazardous solution.

Method

1. Weigh the solid in a sample bottle using a 2 dp balance.

• Record the initial mass of the sample bottle plus the solid.

2. Transfer the solid into a 250 cm³ beaker.

• Re-weigh the empty sample bottle to find the exact mass of the solid used (by difference).

3. Add approximately 100 cm³ of distilled water to the beaker and stir with a glass rod until the solid fully dissolves.
4. Transfer the solution from the beaker to a 250 cm³ volumetric flask using a funnel.

• Rinse the beaker, glass rod, and funnel with distilled water, adding these washings to the volumetric flask.

5. Carefully add distilled water to the volumetric flask until the bottom of the meniscus touches the 250 cm³ mark.

• Use a dropper for the final adjustments to ensure accuracy.

6. Stopper the flask and invert it multiple times to ensure thorough mixing.

Results

• Record the mass of solid used.
• The final volume is always 250 cm³ (if using a 250 cm³ volumetric flask).

Errors and Improvements

• Systematic error from the balance (typically to 0.01 g) is unavoidable but consistent.
• Old or improperly stored anhydrous reagents may absorb moisture, leading to inaccurate mass.
  • Use fresh chemicals or verify the anhydrous nature of the solid.

Reducing Uncertainty

• Use a more accurate balance or a larger mass to minimize percentage error.
• Consider weighing both before and after transferring the solid for increased accuracy (weighing by difference).