Determine unknown sodium hydroxide concentration using known hydrochloric acid and a suitable indicator. This separate-chemistry core practical uses a burette, volumetric pipette, filler, conical flask and white tile.Apparatus and method schematic; not to scale. Use the stated controls and measurements.
A volumetric pipette transfers a fixed volume of alkali accurately. A burette measures the volume of acid delivered. A measuring cylinder is less accurate for transferring this fixed portion (aliquot).
Wear eye protection for acid/alkali. Use a pipette filler, never mouth pipetting; secure the burette upright and use safe filling height.
(Higher tier calculation) For HCl + NaOH → NaCl + H₂O the mole ratio is 1:1. Use n = cV with V in dm³ when concentration is mol/dm³.
Accurate technique
Rinse the burette with the acid and the pipette with alkali after water cleaning. Water left in these measuring instruments dilutes the reagents; the flask can be rinsed with distilled water without changing alkali moles.
Fill the burette including its jet, remove air bubbles and remove the filling funnel. A funnel left in place can drip extra acid after the initial reading.
Transfer a fixed alkali volume to the flask and add a few drops of suitable indicator, such as phenolphthalein or methyl orange. Use the pipette's calibrated drainage procedure; do not blow out a residual drop unless its design requires it.
Read the bottom meniscus of clear liquid at eye level, using the instrument's resolution. Record initial and final readings; titre = final − initial, not the final reading alone.
Perform a rough titration to locate the endpoint, swirling throughout. Near it, add acid dropwise, wash splashes down with distilled water and observe on a white tile.
Repeat accurate titrations until the titres are close together (concordant). School work often uses agreement within 0.10–0.20 cm³, but follow the teacher’s or exam question’s stated limit. Calculate a mean from the concordant accurate runs, leaving out the rough run.
Calculations and evaluation
At the endpoint, record the first persistent specified indicator change. Phenolphthalein changes pink to colourless as acid neutralises alkali; methyl orange changes towards its endpoint colour, not necessarily at pH exactly 7.
The endpoint is the colour change you observe. The equivalence point is where acid and alkali have reacted in exactly the proportions in the balanced equation. Choose an indicator whose colour changes over the steep part of the strong acid–alkali pH curve, close to that point.
(Higher tier) Convert mean acid titre to dm³, calculate acid moles, use the balanced ratio for alkali moles, then divide by pipetted alkali volume in dm³.
Overshooting adds excess acid, inflates titre and gives an overestimated unknown alkali concentration in this calculation. Repeat rather than reverse the addition by guesswork.
An air bubble initially in the jet can make measured titre larger than acid actually entering the flask. Filling and checking the jet targets this systematic error.
Use only concordant suitable titres for a justified mean and state exclusions. Repeated trials with the same wrong acid concentration can be precise but inaccurate.
To prepare pure salt after finding proportions, repeat neutralisation without indicator using the known volumes, then concentrate/cool appropriately. Indicator contaminates a product intended to be pure.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as an air bubble initially left in the burette jet.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom