CP2: A Solid-Acid Standard and Sodium Hydroxide

CP2 prepares a standard solution from a solid acid and uses it to determine sodium hydroxide concentration. Sulfamic acid, NH₂SO₃H, is the context in Pearson’s practical sheet. Its reaction with hydroxide transfers one proton per molecule: NH₂SO₃H + OH⁻ → NH₂SO₃⁻ + H₂O. Check the actual solid formula, purity and molar mass before calculating its concentration.

Weigh the acid accurately, dissolve it fully in a suitable beaker and quantitatively transfer it into a volumetric flask. Rinse the weighing and transfer apparatus into the flask so none of the weighed acid is left behind. Make up to the mark at the appropriate temperature, using dropwise addition near the final meniscus, then stopper and invert to mix. Making a known concentration requires the final solution volume, not simply adding that volume of water.

For the illustrative arrangement here, pipette a known volume of standard acid into a conical flask and add the laboratory-selected suitable indicator. Titrate with the unknown NaOH from a burette. A laboratory may reverse which solution is in the burette; adapt the amount calculation to the actual arrangement. Rinse the pipette with its delivered solution and the burette with its titrant. A little deionised water in the conical flask changes concentration but not the acid amount.

Use a rough run to locate the end point, then accurate runs with swirling and dropwise addition near the permanent colour change. Record initial and final burette readings and apply the stated concordance criterion. Fill the burette safely below eye level with eye protection, remove the funnel before titrating and use a pipette filler.

Calculate standard acid moles from cV, use the 1:1 reaction ratio, then divide the hydroxide amount by its delivered volume. Do not import the carbonate–HCl 1:2 ratio: this solid acid supplies one proton. Uncertainty in weighed mass, flask volume, pipette volume and titre can all contribute.

Overshooting the end point uses too much NaOH volume for the assigned acid amount, making its calculated concentration too small in this arrangement. Losing solid during transfer makes the actual acid concentration smaller than the nominal value calculated from the original mass. A consistent error can affect every concordant run. Sodium hydroxide also changes on exposure to air, which is one reason standardisation and suitable storage matter.

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