Concentration Calculation for a Dilution Step
A laboratory technician must prepare 500 mL of a 0.10 mol/L sodium hydroxide solution for a titration practical. The stockroom holds a concentrated NaOH solution labelled 2.0 mol/L. The technician also needs 250 mL of 0.050 mol/L NaOH for a second group. Glassware available: a 500 mL volumetric flask, a 250 mL volumetric flask, a 50 mL measuring cylinder, a 10 mL graduated pipette, and a 100 mL beaker. Distilled water is available. The technician is told to use the smallest volume of stock that meets both requirements without preparing excess.
1. Calculate the volume of 2.0 mol/L stock needed for each solution and the total stock volume required.
Read the worked answer
Use the dilution relation C1V1 = C2V2.
For the 500 mL of 0.10 mol/L solution: V1 = (0.10 × 500) / 2.0 = 50 / 2.0 = 25 mL of stock.
For the 250 mL of 0.050 mol/L solution: V1 = (0.050 × 250) / 2.0 = 12.5 / 2.0 = 6.25 mL of stock.
Total stock required = 25 + 6.25 = 31.25 mL.
The 50 mL measuring cylinder can deliver the 25 mL accurately enough, and the 10 mL graduated pipette can deliver the 6.25 mL. Note that the 25 mL aliquot cannot be measured with the 10 mL pipette in a single step without introducing cumulative error, so the cylinder is the better choice for that portion.
2. Describe the correct procedure for making the 500 mL solution, naming the glassware used at each stage.
Read the worked answer
Measure 25 mL of 2.0 mol/L stock with the 50 mL measuring cylinder and transfer it to the 500 mL volumetric flask. Rinse the cylinder with a little distilled water and add the rinsings to the flask so no solute is lost.
Add distilled water to about two-thirds of the flask volume and swirl to mix. Then add water dropwise until the bottom of the meniscus sits on the 500 mL calibration mark, with the eye level with the mark. Stopper and invert the flask several times to homogenise.
The 100 mL beaker is not needed for this dilution and should not be used as a mixing vessel, because volume is only accurate in the volumetric flask. The 250 mL solution is prepared the same way using the 10 mL pipette for 6.25 mL and the 250 mL volumetric flask.
3. Identify one handling hazard of concentrated NaOH and state the precaution that directly controls it.
Read the worked answer
Concentrated NaOH is strongly alkaline and causes severe chemical burns and eye damage on contact, so the direct control is personal protective equipment: splash-proof goggles, a face shield where splashing is likely, nitrile gloves, and a laboratory coat.
Because dilution of alkali is exothermic, add the stock to water rather than water to stock, and do it slowly with swirling to limit heat and spattering. Keep a eyewash station accessible and know its location before starting. Any spill on skin should be flushed with plenty of water for at least 15 minutes and reported to the supervisor.