Question

10) A student prepares aspirin (acetylsalicylic acid, C,H,0.) by reacting salicylic acid (C,H,O,) with acetic anhydride (C.H.O.). The balanced equation for the reaction is: 2 C,H,O, + C,H,O, + 2 C,H,O, + H,O The lab calls for 3.00 g of salicylic acid to be mixed with 6.40 g of acetic anhydride. What is the limiting reagent and what is the maximum amount of aspirin which can be prepared if the yield for the reaction is 91%?

Answer 1

1)

Molar mass of C7H6O3,

MM = 7*MM(C) + 6*MM(H) + 3*MM(O)

= 7*12.01 + 6*1.008 + 3*16.0

= 138.118 g/mol

mass(C7H6O3)= 3.0 g

use:

number of mol of C7H6O3,

n = mass of C7H6O3/molar mass of C7H6O3

=(3 g)/(1.381*10^2 g/mol)

= 2.172*10^-2 mol

Molar mass of C4H6O3,

MM = 4*MM(C) + 6*MM(H) + 3*MM(O)

= 4*12.01 + 6*1.008 + 3*16.0

= 102.088 g/mol

mass(C4H6O3)= 6.4 g

use:

number of mol of C4H6O3,

n = mass of C4H6O3/molar mass of C4H6O3

=(6.4 g)/(1.021*10^2 g/mol)

= 6.269*10^-2 mol

Balanced chemical equation is:

2 C7H6O3 + C4H6O3 ---> 2 C9H8O4 + H2O

2 mol of C7H6O3 reacts with 1 mol of C4H6O3

for 2.172*10^-2 mol of C7H6O3, 1.086*10^-2 mol of C4H6O3 is required

But we have 6.269*10^-2 mol of C4H6O3

so, C7H6O3 is limiting reagent

Answer: C7H6O3

2)

we will use C7H6O3 in further calculation

Molar mass of C9H8O4,

MM = 9*MM(C) + 8*MM(H) + 4*MM(O)

= 9*12.01 + 8*1.008 + 4*16.0

= 180.154 g/mol

According to balanced equation

mol of C9H8O4 formed = (2/2)* moles of C7H6O3

= (2/2)*2.172*10^-2

= 2.172*10^-2 mol

use:

mass of C9H8O4 = number of mol * molar mass

= 2.172*10^-2*1.802*10^2

= 3.913 g

% yield = actual mass*100/theoretical mass

91.0= actual mass*100/3.913

actual mass=3.561 g

Answer: 3.56 g