AQA AS Level Chemistry - Required Core Practical Summary Notes (Methodology, Analysis, Results and Observations)
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Course
Core Practicals
Institution
AQA
This document is a detailed and extensive guide for all of the 6 required practicals for AQA AS level chemistry. Revising the core practicals for exams can be difficult, so I prepared a resource which not only contains the method for each practical but also highlights where specific equipment/techn...
1. Making up a Volumetric Solution
Method
1) Calculate the mass of solid needed to produce 250cm3 of a 0.1 mol dm-3 solution.
2) Weigh the mass of an empty, clean weighing boat on a 2d.p. balance.
3) Add the calculated mass of solid into the weighing boat using a spatula and reweigh.
4) Pour the contents of the weighing boat into a 250cm3 beaker and reweigh.
The difference in mass is an accurate measurement of the mass of solid that is added to the beaker.
5) Add approx. 100cm3 of distilled water to the beaker and stir using a glass rod until all of the solid
dissolves.
It does not matter if the volume of water added slightly exceeds 100cm 3, as the volume must be made
up to 250cm3 in the end.
Substances may not dissolve well in cold water, in which case the contents could be heated gently.
6) Using a funnel, pour the contents of the beaker into a 250cm3 volumetric flask and rinse the glass rod
/ beaker contents into the flask.
Rinsing the glassware ensures that all of the salt is transferred into the volumetric flask.
7) Carefully add distilled water to the volumetric flask up to the graduated line.
Use a dropping pipette for the last few drops to ensure you don’t overshoot the graduated mark.
Fill until the bottom of the meniscus sits on the graduated line.
8) Stopper the flask.
A stopper will prevent water from evaporating from the flask. If water evaporated from the flask, the
concentration of the solution will increase.
9) Invert the flask to mix the contents.
Only invert the flask after reaching the graduated line, otherwise water will remain on the edges of
the flask and gradually add to the solution, reducing the concentration.
Inverting will ensure that the concentration of the substance is uniform throughout the solution.
Potential Sources of Error
• If using an anhydrous salt, make sure it has not picked up water otherwise its mass values will not be
accurate.
• Balance only gives values to 0.01g.
• With dark liquids, it can be difficult to see the meniscus – place a piece of white paper behind the
marker clearer to see.
Reducing Uncertainty in Measuring Mass
• Use a balance with a higher resolution.
• Use a larger mass of solid.
• Weigh the sample before and after the addition of the solid to the beaker (weigh by difference).
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