In: Chemistry
4- Aspirin can be made in the laboratory by reacting acetic anhydride (C4H6O3) with salicylic acid (C7H6O3) to form aspirin (C9H8O4) and acetic acid (HC2H3O2). The balanced equation is:
C4H6O3 + C7H6O3 → C9H8O4 + HC2H3O2
In a laboratory synthesis, a student begins with 3.00 mL of acetic anhydride (density = 1.08 g/mL) and 1.25 g salicylic acid. Once the reaction is complete, the student collects 1.22 grams of aspirin.Determine the limiting reactant, theoretical yield of aspirin, and percent yield for the reaction.
5. Combustion analysis of a 13.42 g sample of equilin (which contains only carbon, hydrogen and oxygen) produces 39.61 g of CO2 and 9.01 g of H2O. The molar mass of equilin is 268.34 g/mol. Find the molecular formula of equilin.
Answer 4)
A stoichiometrically balance reaction for synthesis of Aspirine
C7H6O3 + C4H6O3 -------> C9H8O4 + HC2H3O2
Salicylic acid Acetic anhydride Aspirine Acetic acid
This means 1 mole of salicylic acid react copletely with 1 mole of Acetic anhydride to give 1 mole of Aspirine.
I.e.
1 mole of Salicylic acid 1 mole of Aceetic anhydride 1 mole of Aspirine
Using molar mass values we write, mas equivalence relation as
138.1 g of Salicylic acid 102.1 g of Aceetic anhydride 180.2 g of Aspirine.
Now in given experiment,
Volume of acetic anhydride = 3.00 mL density = 1.08 g/mL
Density = Mass / volume
Mass = Density x volume
Mass of acetic anhydride = 1.08 x 3.00 = 3.24 g
And Mass salicylic acid = 1.25
From mass equivalence relation it's clear that mass of salicylic used must be breater that mass of acetic anhydride for complete reaction of salicylic.
But in given experiment mass of salicylic (1.25 g) used is less than mass of acetic anhydride (3.24)
hence it's obvious that Salicylic acid is LIMITING Reagent.
Hence Theoretical Yield will be calculated with respect to mass of salicyic acid.
a) Theoretical Yield calculation:
Mass equivalence equation says
If, 138.1 g of Salicylic acid 180.2 mole of Aspirine.
Then 1.25 g of Salicylic acid say 'A' g of Aspirine.
On cross multiplication,
A x 138.1 = 180.2 x 1.25
A = 180.2 x 1.25 / 138.1
A = 1.63 g.
Hence Theoretical i.e. expected yield (100 % yield) of aspirine = 1.63.
b) % yield of aspirine.
% yield = (Actual experimental yield / Theoretical Yield) x 100
Actual yield of Aspirine = 122.0 g
so,
% yield of Aspirine = (Actual experimental yield of Aspirine / Theoretical Yield of Aspirine) x 100
= (1.22 / 1.63) x 100
= 74.85
% yield of Aspirine = 74.85 %.
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Answer 5)
Given that an Equilin contain only C, H, O.
Let there are x,y and z # of C atoms in 1 molecule of equilin hence M.F. of equiline be CxHyOz
Combustion of equilin can be given as,
CxHyOz + n O2 -------> xCO2 + y/2 H2O.
It means 1 mole of equiline gives x moles of CO2 and y/2 moles of H2O.
Let us calculate # of moles of equilin using given mass and molar mass of equiline .
# of moles of Equiline = mass of equiln burnt Molar mass = 13.42 / 268.34 = 0.05
hence 0.05 moles of equiline expected to give 0.05x moles of CO2 and 0.05 (y/2) moles of H2O.
# of moles of CO2 formed = mass of CO2 formed / molar mass = 39.61/44 = 0.9
i.e. # of moles of CO2 formed = 0.05 x = 0.9
so, x = 9/5 = 18
then,
# of moles of H2O = 9.01 / 18 = 0.51
i.e. # of moles of H2O = 0.05 (y/2) = 0.51
y = 1.02 / 0.05 = 20.4
y = 20 (approx)
From assumed formula of equlin we get,
Molar mass of equlin = 12 x (x) + 1 x y + 16 x z = 268.34
using x and y values,
12 x 18 + 1 x 20 + 16 z = 268.34
16 z = 268.34 - 236
16 z = 32 (approx.)
z = 32/16 = 2
With x = 18, H = 20, z = 2 we get the molecular formula for equlaine as,
C18H20O2.
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