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Summary required practical 1: method and explanation of uncertainty
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explanation of how to make a standard solution and how to carry a titration - full method
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Bonding Shapes Summary AQA A Level Chemistry Unit 3.1.3 - Bonding
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Required practical 1Making a standard solution
A standard solution is a solution for which the concentration is
accurately known
When making a standard solution a larger mass of solid dissolved
gives a smaller weighing error, so will be a more accurate solution
Sodium hydroxide is not a suitable solid for making up a standard
solution as it absorbs water from the atmosphere
Method
1. Weigh out an accurate mass of solid in a clean, dry beaker
2. Add enough deionised water to dissolve the solid, stirring with a
glass rod
3. Transfer the solution with rinsing to a 250cm volumetric flask using
a funnel
4. Rinse the beaker and glass rod with deionised water and add to the
volumetric flask
5. Add deionised water up until the bottom of the meniscus
6. Insert bung and invert and shake to mix thoroughly
Uncertainty in mass measurements
Uncertainty is an estimate attached to a measurement that gives the
range of values within which the true value is thought to lie
Percentage error = uncertainty / quantity measured x100
The more decimal places a balance reads to, the smaller the percentage
error
If two mass measurements are taken, then the formula for the %
uncertainty is:
% uncertainty = 2 x uncertainty in each measurement / quantity
measured x100
Titration method
1. Rinse the burette with the solution you are going to fill it with,
discard the rinsings and fill the burette
2. Rinse a pipette with the solution you are going to put in the conical
flask. Using a pipette and pipette filler transfer 25.0 cm of this
solution in to a conical flask
3. Add 2-3 drops of indicator
4. Add the solution from the burette, with constant swirling of the
conical flask until a colour change. This is the practice titration
5. Repeat 2-3 time until you have concordant results, adding the
solution dropwise near the end point
6. Calculate the mean titre of concordant results
*remove funnel from burette before titrating
* fill the burette tap before you start titration
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