DPST1031 Chap.13 Laboratory Core Skills and Chemical Analysis
Laboratory Core Skills and Chemical Analysis
The laboratory program is worth 20% of the course and carries the one hurdle that can fail a student outright: every core laboratory skill must be awarded by the end of term. This chapter explains how the hurdle and graded halves of the laboratory mark work, which skills are core and which are graded, and the attendance and pre-lab rules.
It then walks through the techniques the core skills check, weighing by difference, preparing a standard solution in a volumetric flask, pipetting and titrating to concordant results, and works the standardisation, standard solution and dilution calculations that every report needs.
What this chapter covers
- 01
Core and non-core laboratory skills
- 02
How the hurdle and graded marks combine
- 03
Attendance, make-up labs and pre-lab work
- 04
Weighing by difference
- 05
Preparing a standard solution
- 06
Pipette and burette technique
- 07
Concordant titres, accuracy and precision
- 08
Standardisation and dilution calculations
Worked example · free
Choosing which titres to average
- +1The rough titre only locates the endpoint, and 24.30 mL is 0.60 mL above the others.
- +1Average the three accurate titres: (23.65 + 23.75 + 23.70) ÷ 3 = 23.70 mL.
- +1Each accurate titre lies within 0.05 mL of 23.70 mL, inside the 0.2 mL concordance limit.
- +1Use 23.70 mL; with 25.00 mL of 0.1052 M HCl, this gives NaOH at 0.1110 M.
Key terms
- Core laboratory skill
- A required skill that must be awarded for the laboratory component, and the course, to be passed.
- Weighing by difference
- Finding the mass transferred as the loss in mass of a sample tube on an analytical balance.
- Standard solution
- A solution whose concentration is known accurately from a weighed pure solid and a measured volume.
- Volumetric flask
- A flask calibrated to hold one exact volume at its graduation mark.
- Titre
- The volume of titrant delivered from the burette to reach the endpoint.
- Concordant results
- Repeated titres that agree within a stated tolerance, 0.2 mL of their average in this course.
- Endpoint
- The point in a titration at which the indicator changes colour.
Laboratory Core Skills and Chemical Analysis FAQ
What happens if I miss a core laboratory skill?
Core skills are all or nothing. If any core skill has not been awarded by the end of term, you fail the laboratory component and therefore the course. There are several chances to show each skill, and make-up laboratory classes give a final opportunity, so use them for any skill still outstanding.
How are laboratory marks split?
In the Term 3 Moodle table the laboratory is worth 20% in total: 10% for the hurdle tasks, awarded in one lump once every core skill is achieved, and 10% for graded tasks, built up from non-core skills such as accuracy, calculations and observations across the experiments.
Why rinse a pipette with the solution but not a volumetric flask?
A pipette delivers the liquid inside it, so any water left in it would dilute the sample and change the amount delivered. A volumetric flask will be filled with water to the mark anyway, so it only needs to be clean, not dry or rinsed with the solution.
What makes titres concordant?
In this course, titres are concordant when each lies within 0.2 mL of their average. The rough first titre is used only to locate the endpoint and is normally excluded. Three concordant accurate titres are then averaged to give the volume used in the calculation.
Why does an overshot endpoint affect accuracy but not precision?
If the endpoint is overshot by a similar amount every time, the titres still agree closely with each other, so the results are precise. They are all too large, however, so the calculated concentration is wrong and the result is inaccurate. Approaching the endpoint dropwise prevents this systematic error.
Exam move
Before each laboratory class, complete the pre-lab work and safety information, watch the technique video for the skill being assessed, and read the feedback criteria for that experiment so you know exactly what earns each skill. Practise the calculations at home first: moles from mass, concentration from moles and volume, dilution, and titration stoichiometry including 1 : 2 ratios.
In the class, record initial and final burette readings for every run and the actual mass weighed. After each class, check which core skills are still missing and plan to collect them before the make-up laboratory weeks.
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