Skill 1 - Planning Charlotte Nellist
‘The Molecular Formula
of Succinic Acid’
The aim of this experiment is to determine the molecular formula of a sample of anhydrous succinic acid. Succinic acid is a diprotic, which means it donates two protons per molecule. Succinic acid can be completely neutralised by sodium hydroxide. The indicator most suitable for this experiment is phenolphthalein, it is colourless in acids and pink in alkalises. The half way stage is about pH 9.3, this is when it will either change from colourless to a very pale pink or from pink to colourless. To determine the relative formula mass of succinic acid I am going to do a titration against sodium hydroxide.
The equation for the reaction is given below. To make the equation easier to read, HOOC(CH2)nCOOH will be condensed to H2A because of the two hydrogen atoms at either end.
H2A+2NaOH → Na2A+2H2O
(CV) H2A = 1
(CV) NaOH 2
I am going to use the NaOH as 0.1M because I don’t want it too concentrated, so therefore I am going to use H2A as 0.05M because of the ratio 2:1.
In the formula of succinic acid below n is a whole number between 1 and 4. So therefore first I need to calculate the relative molecular mass of succinic acid.
HOOC(CH2)nCOOH H = 1 O = 16 C = 12
Mr when n = 1 1+16+16+12 (12+2) 12+16+16+1
= 104
Mr when n = 2 1+16+16+12 [(12+2) x2] 12+16+16+1
= 118
Mr when n = 3 1+16+16+12 [(12+2) x3] 12+16+16+1
= 132
Mr when n = 4 1+16+16+12 [(12+2) x4] 12+16+16+1
= 146
From these calculations I can see that I need between 104g and 146g in 1 litre to equal 1M. But I want the solution in 250cm3, so therefore I need to divide the weights by 4:
n = 1 104 = 26g So I need between 26g and 36.5g in 250cm3 to make a 1M
4 solution.
n = 4 146 = 36.5g
4
I also want to make the solution to 0.05M because of the ratio 2:1, so therefore I need to multiply each weight by 0.05.
n = 1 26 x 0.05 = 1.3g
n = 4 36.5 x 0.05 = 1.8205g
So the range I can work with to weigh out the anhydrous succinic acid is from 1.3g to 1.82g,which will make a 0.05M solution in 250cm3.
Preparing a standard solution
Having calculated the weight I can use (1.3g-1.82g), I must weigh out the solute using an accurate electronic balance that goes to three decimal places. I must make sure I clean the balance with a fine brush assuming that it may not have been cleaned after the last time it was used and set the balance back to 0.00. I must also make sure I careful when using the spatula to add the white crystalline solid anhydrous succinic acid. I then need to clean a 250cm3 beaker with distilled water and then add 50cm3 of distilled water to it; I will then carefully transfer the bulk of solute into the beaker stirring until all the solute has dissolved. This shouldn’t take long as the sample of succinic acid is anhydrous and very soluble in water. To make sure I have all the acid I will wash the weighing bottle with distilled water and add it to the beaker, I will do this three times just to be certain I have all the acid. I will carry on stirring with a glass rod (which will have also been washed in distilled water before use) and adding more distilled water if necessary until all the solute has dissolved. On completing that I will wash a 250cm3 volumetric flask out with distilled water. Then I need to transfer all the solution into the volumetric flask. I will do this using the aid of a funnel (which will have also been washed in distilled water). After all the solution has been transferred, I will wash the beaker with distilled water and transfer all the washings to the volumetric flask, I will carry out this three times, and then wash the funnel through, again repeating this several times. I will at that time add more distilled water to the volumetric flask until the water level is 1cm from the 250cm3 mark on the neck. After that I will place the stopper over the top and turn the flask upside down and shake the contents, returning it upright. I will do repeat this ten times to again to be certain all the solute has dissolved. Next using a dropping pipette (which will have also been cleaned with distilled water) I add distilled water slowly to bring the meniscus up to the flask. I will be at eye level with the flask at this point to make sure it is as accurate as I can get it. I will then again place the stopper on the flask and turn it upside down and shake the contents, returning it upright, again repeating this ten times.