Question

i cant get the weight of aspirin

The Standard
Concentration of Aspirin Standard 2.551 mg/mL
Volume of Aspirin Standard used 3.59 mL
Diluted to 100.00 mL (volumetric flask) with FeCl3 solution.
This is Standard 1
Using the same cuvet for all measurements, the following %T's were obtained:
blank 100.7 %
Standard 1 27.4 %
The Unknown
Weight of tablet 498.4 mg
Dissolved at boiling point in dilute NaOH. Quantitatively transferred and diluted to 100.00 mL.
Volume of original tablet solution diluted to 100 mL with FeCl3 solution. 2.33 mL
%T unknown 21.7 %
Calculate:
a) The Concentration of Aspirin in Standard 1 _.09158______________ mg/mL
b) The absorbance of Standard 1 ________.565_______
c) Absorbance of unknown solution _______.665________
Concentration of aspirin in tablet solution
d) as measured in cuvet __________.1080_____ mg/mL
e) concentration in original tablet solution _______4.6835________ mg/mL
f) Total weight in mg of pure Aspirin in tablet _______________ mg

Answer 1

Lets do this in a stepwise manner,

Aspirin which is 2-acetylsalicylic acid will give a -ve test with FeCl3. However, any unreacted salicyclic acid would react with FeCl3 to give a +ve FeCl3 test.

The reaction of salicycli acid with FeCl3 being,

3C7H6O3 (aq) + FeCl3 (aq) ------> Fe(C7H5O3)3 (aq) + 3HCl (aq)

The iron (III) salicylate is a compound of iron that is deeply colored in solution and this method can be used to qualitatively, or even quantitatively to detect salicyclic acid (or salicylate salts) in solution.

So, 3 moles of salicylic acid would react with 1 mole of FeCl3.

Given here,

The Standard

Concentration of Aspirin Standard 2.551 mg/mL

Volume of Aspirin Standard used 3.59 mL

Diluted to 100.00 mL (volumetric flask) with FeCl3 solution.

This is Standard 1

a) Lets find the concentration of aspirin in 100 mL diluted solution.

Using, C1V1 = C2V2

where,

C1 = 2.551 mg/ml

V1 = 3.59 mL

C2 = unknown

V2 = 100 mL

Feed the values,

2.551 x 3.59 = M2 x 100

C2 = 0.0916 mg/ml is the concentration of aspirin in 100 mL solution.

b) We know,

Absorbance (A) = 2 - log(%transmission (T))

So absorbance of standard 1 = 2 - log(27.4) = 0.562

c) Using the same formula as above,

Absorbance of unknown = 2 - log(21.7) = 0.664

Now calculate concentration of aspirin

Aspirin on reaction with dil NaOH hydrolyze the acetyl group to free OH group,

C9H8O4(aq) + NaOH(aq) -----> C7H6O3(aq) + H2O(l)

The free OH then reacts with FeCl3 as shown above

498.4 mg aspirin tablet is hydrolyzed and diluted to 100 mL

2.33 ml of this solution is taken and rediluted to 100 mL with FeCl3 solution

Concentration of Aspirin in 2.33 ml = 498.4 mg/2.33 ml = 213.906 mg/ml

Diluted to 100 mL

Concentration of aspirin in diluted 100 mL

C1V1 = C2V2

where,

C1 = 213.906 mg/ml

V1 = 2.33 mL

C2 = unknown

V2 = 100 mL

Feed values,

213.906 x 2.33 = C2 x 100

C2 = 4.984 mg/ml

d) as measured in cuvette = 4.984 mg/ml

e) Concentration of original tablet solution in 2.33 mL = 213.906 mg/ml

f) Total weight in mg of pure aspirin in tablet

Since standard 1 has absorbance = 0.562 for 2.551 mg/ml of aspirin

Thus, absorbance of unknown measured as 0.664 will be for 3.014 mg/ml

concentration of aspirin in 100 mL = 3.014 x 100 = 301.4 mg

So, the total weight in mg of pure aspirin in tablet = 301.4 mg