Chemical amounts (A2)
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How can you determine the molecular formula and molar mass of succinic acid, by doing a titration with an appropriate standard solution of sodium hydroxide with a ratio of one mole of acid to two moles of succinic acid? How can a knowledge of the relative formula mass be used to calculate the molecular formula of succinic acid?
Rev replies ...
It is clear that you know quite a lot about the acid to start with. You know it has two acid group with hydrogen atoms that can ionise. In other words it is a diprotic acid. You probably also know that it is an organic acid and so a weak acid
If we think of the acid as H2A the reaction will be
H2A + 2 NaOH -> Na2A + H2O
Thus 1 mol of the acid reacts with 2 mol sodium hydroxide. If we can find the mass of acid that reacts with a known amount (in moles) of sodium hydroxide we can then work out the amount in moles of the acid. Knowing the mass and the amount in moles allows the molar mass to be calculated.
Assume you have standard 0.1 M NaOH.
You need to make an informed guess as to the amount of acid to weigh out to make a standard solution so that you get a reasonable titration. About 2.5 to 3.5 g in 250 cm3 seems about right. Make a solution as usual and then titrate a sample (25 cm3) with the standard alkali choosing an indicator for a weak acid/strong base titration.
Results
Mass of acid used to make the solution = y g
Volume of 0.1M NaOH used z cm3
z cm3 of the 0.1M NaOH contain z x 0.1 / 1000 moles of NaOH
Therefore 25 cm3 of your solution of H2A contain half this number of moles of H2A. Let's take this answer to be p .
Therefore 250 cm3 of your solution of H2A contain 10p moles
Therefore y grams of H2A is 10p moles of H2A
You know how many moles of H2A there are in y grams of solid you have now to find out what mass would be one mole. This is the molar mass.
Now the formula.
You know it is organic.
You know that organic acids have -COOH groups.
You know this acid has two of these.
These two groups will have a mass of 2 x (12+32+1) g.
Take this off the molar mass and see what’s left.
The rest of the acid is probably made up of carbons and hydrogens so you can work out from what is left the number of CH2 groups and use the fact that carbon has a valency of four to make an intelligent guess at the structure.
That mass. You know it has two COOH groups so you know the minimum molar it could have which allows you to estimate the mass you need for a 0.1M solution and add a bit on!
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updated: 07 November 2003
