PAG2: Standard Solutions and Acid–Base Titration

A primary standard has suitable purity and stability for a concentration calculated from its weighed mass. Sodium hydroxide pellets absorb water and carbon dioxide, so weighing pellets alone does not establish an accurate standard concentration. Anhydrous sodium carbonate is a common solid standard: calculate its amount using the stated formula and purity.

Weigh by difference when appropriate, dissolve completely in a beaker, and transfer through a funnel into a volumetric flask. Rinse the vessel, stirring rod and funnel into the flask so the measured amount reaches it. Make up the solution near the mark, then add water dropwise with the meniscus at eye level. Mix by repeated inversion after stoppering. Allow a warm solution to reach the calibration temperature before making up to volume.

Rinse the burette with titrant and the volumetric pipette with the solution it will deliver; residual water in either would dilute it. Use a pipette filler. Fill the burette tip, remove air bubbles, remove the filling funnel and record readings consistently with the instrument scale. The burette need not start at zero: delivered volume is final minus initial reading.

A conical flask may be rinsed with deionised water because this changes concentration but leaves the pipetted amount unchanged. Use a suitable indicator and a rough titration to locate the end point. For accurate runs, swirl and add titrant dropwise near the permanent colour change. Repeat until the specified concordance criterion is met; do not include the rough run in a mean just to increase sample size.

Find the mean of the concordant titres, calculate moles of known solution, apply the balanced-equation ratio and divide by the unknown aliquot volume in dm³. For carbonate neutralised fully by hydrochloric acid, one carbonate reacts with two HCl. State any dilution factor separately.

For an illustrative burette uncertainty of ±0.05 cm³ per reading, a titre uses two readings, giving a conservative absolute uncertainty of ±0.10 cm³. Its percentage contribution decreases for a larger titre. Concordance supports precision; a consistent wrong concentration or overshot end point can still bias every run. Match an improvement to the error rather than claiming that repeating removes systematic error.

A suitable indicator changes within the steep part of the chosen titration curve. Methyl orange is suitable for full carbonate neutralisation by strong acid; phenolphthalein responds at a different carbonate stage and must not be treated as interchangeable with the same reaction ratio. Weak acid–strong base and strong acid–weak base titrations also need their own indicator justification. The aliquot, indicator and balanced equation must describe the same end point.

An OCR centre may choose a different suitable standard and chemical pair. The quantitative-transfer and volume techniques remain the same, but recalculate the acid’s proton stoichiometry and hydration formula rather than reusing the carbonate numbers unchanged.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.