Convert solution units and explain reliable volumetric measurements.
Method and worked examples
Use c=Vn with V in litres. Mass concentration in gL−1 equals c times molar mass. A 1% vw solution contains 1g per 100mL, or 10gL−1. For dilute water of density 1.00kgL−1, mg L−1 approximately equals ppm and µgL−1 approximately equals ppb.
A pipette delivers an aliquot; a burette measures final minus initial reading. Rinse each with its working solution after cleaning to avoid dilution by residual water. Distilled water in a conical flask changes volume but not analyte moles. Concordant titres show precision; a shared systematic error can still reduce accuracy. The observed end-point approximates the stoichiometric equivalence point.
Example 1
Convert 0.200 mol L−1 NaOH to gL−1 using M=40.0gmol−1.
Mass concentration =cM=0.200×40.0=8.00gL−1.
Example 2
A dilute water sample contains 2.5 mg solute per litre and has density 1.00kgL−1. Express this in ppm.
One litre has mass 1.00kg, so 2.5 mg kg−1=2.5 ppm. The density assumption makes mg L−1 and ppm approximately equivalent.
Watch out:It dilutes the flask contents but does not change analyte moles; the same titrant amount is required if the end-point remains suitable.
Concentration units and titration technique | Year 12 Chemistry | Ascent Arcade