Quantitative Analysis (1)
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Define titration.
an experiment that lets you see what volume of a reactant is needed to react completely with a certain volume of another reactant
Describe how you would carry out an acid-alkali titration. (12 steps)
• Use a pipette with a pipette filler to measure out a set volume of alkali. • Put the alkali in a flask with a few drops of indicator. • Using a funnel, fill a burette with some acid of a known concentration and measure the starting volume. • Put the flask containing the alkali under the burette. • First, do a rough titration. Add the acid to the alkali a bit at a time, giving the flask a regular swirl. • When the indicator changes colour, calculate the amount of acid that was needed to neutralise the alkali. This is your rough titre. • Now, do an accurate titration. Take an initial reading of how much acid is in the burette. • Add acid until you are within 2ml of the end point. • Continue to add the acid drop by drop until you observe the indicator colour change. • Calculate the volume of acid used to neutralised the alkali. • Repeat this accurate titration until you have at least three concordant results (within 0.1ml). • Calculate the mean volume of acid that was needed to neutralise the alkali using your concordant results.
Why isn’t universal indicator suitable for titrations?
it changes colour too gradually
What equation links concentration, moles and volume?
moles = concentration x volume n = CV n: mol C: mol/dm^3 V: dom^3
What equation allows you to convert concentration from mol/dm^3 and g/dm^3?
C (g/dm^3) = C(mol/dm^3) x RFM
Define theoretical yield.
the amount you would get if all the reactants formed the desired products and none of the products were lost
Define yield.
the actual amount of product you actually get in reaction
What is percentage yield? and how is this calculated?
it is a comparison between the yield you actually get and the theoretical yield it is calculated using the equation: percentage yield = actual yieldx100 / theoretical yield
What are the main reasons why percentage yields are never 100%? (3 reasons with a brief explanation)
• The reaction is incomplete - not all of the reactants are converted to products. • Product is lost during practical procedures - when transferring liquids, some of it always gets left behind on the surface of the old container. Also, when filtering a solid from a liquid you always lose a bit of the liquid or solid. • Unexpected reactions may be happening - sometimes, unexpected side-reactions occur.
Define atom economy.
in a reaction, what percentage of the mass of the reactants ends up as a desired product
What equation allows you to calculate atom economy?
atom economy = total RFM of desired productsx100 / total RFM of all reactants
What are the negatives of reactions with low atom economies? (3)
• they use up resources very quickly • they make lots of waste materials that have to be disposed of • they aren’t very profitable as raw materials can be expensive and waste products can be expensive to dispose of properly
What are some important factors that need to be taken into account when choosing reactions in industry? (3)
• percentage yield • rate of reaction • whether or not the reaction is reversible
Define molar volume. and state what it is measured in.
the volume occupied by one mole of gas this is measured in dm^3 / mol
What equation allows you to calculate the molar volume of a gas?
molar volume = gas volume / number of moles
What equation links gas volume and moles?
gas volume = moles x 24 V = n x 24 V: dm^3 n: mol