Basic Laboratory Techniques and Measurements
Candidates must be able to perform and explain core techniques with correct units, significant figures and precision. Understand molarity (moles per litre), percentage solutions (weight per volume and volume per volume), dilution using the relationship C1V1 = C2V2, and serial versus simple dilution. Know how to read a meniscus at eye level, how to use a pipette correctly (pre-rinse, vertical, slow aspiration and delivery, touch-off), and how to record measurements to the precision of the instrument. Understand that accuracy is closeness to the true value, while precision is reproducibility; random error affects precision, systematic error affects accuracy.
Traps: mixing units (mL and L) in dilution calculations; rounding too early and losing significant figures; assuming a 10-fold serial dilution equals a single 10-fold dilution in total volume; using a measuring cylinder when a volumetric pipette is required for accuracy. Remember that temperature affects volume and density, so standard solutions are prepared at a stated temperature. Understand the purpose of a blank, a control and a replicate.
Worked example: To prepare 100 mL of 0.1 M sodium chloride from a 1.0 M stock, use C1V1 = C2V2: V1 = (0.1 x 100) / 1.0 = 10 mL of stock, made to 100 mL with diluent. A common trap is adding 10 mL stock to 100 mL diluent, giving 110 mL total and the wrong concentration.
Rule of thumb: make to the final volume, not add to it.