Problem: Researchers obtained the following data from experiments to find the molecular formula of benzocaine, a local anesthetic, which contains only carbon, hydrogen, nitrogen, and oxygen. Complete combustion of a 3.54-g sample of benzocaine with excess O2 forms 8.49 g CO2 and 2.14 g H2 O. Another 2.35 g sample contains 0.199 g of N. The molar mass of benzocaine is 165 g/mol.Find the molecular formula of benzocaine.

FREE Expert Solution

Step 1: Determine the mass of C and H produced from the reaction


So the mass of C from 8.49 g CO2 is


8.49 g CO2(1 mol CO244.01 g CO2)(1 mol C1 mol CO2)(12.01 g C1 mol C) = 2.3169 g C


So the mass of H from 2.14 g H2O is


2.14 g H2O(1 mol H2O18.016 g H2O)(2 mol H1 mol H2O)(1.008 g H1 mol H) = 0.2395 g H


Step 2: Determine the mass percent of C, H and N


Recall that mass percent is given by:

mass percent = mass of Xtotal mass×100


% C = 2.3169 g3.54 g×100= 65.4492 % C


% H = 0.2395 g3.54 g×100= 6.7655 % H


% N = 0.199 g2.35 g×100= 8.4681 % N


Step 3: Determine the mass and moles C, H, O and N from their mass percent


%C + %H + %O + %N = 100%

%O = 100% – (% C + % H + %N) 

%O = 100% - (65.4492 %C + 6.7655% H + 8.4681% N)

%O = 19.3172 % O


Assuming we have 100 g of the compound, this means we have 65.4492 g C, 6.7655 g H, 8.4681 g N and 19.3172 g O


65.4492 g C ×1 mol C12.01 g C5.4496 mol C



6.7118 g H×1 mol H1.008 g H = 6.6585 mol H


8.4681 g N×1 mol N14.01 g N = 0.6044 mol N


19.3172 g O×1 mol O16.00 g O = 1.2073 mol O



Step 4: Determine the lowest whole number ratio of C, H, O and N to get the empirical formula


5.4496 mol C0.6044 mol9 C


6.6585 mol H0.6044 mol11 H


0.6044 mol N0.6044 mol1 N


1.2073 mol O0.6044 mol2 O


The empirical formula of benzocaine is C9H11NO2


Step 5: Determine the molecular formula


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Problem Details

Researchers obtained the following data from experiments to find the molecular formula of benzocaine, a local anesthetic, which contains only carbon, hydrogen, nitrogen, and oxygen. Complete combustion of a 3.54-g sample of benzocaine with excess O2 forms 8.49 g CO2 and 2.14 g H2 O. Another 2.35 g sample contains 0.199 g of N. The molar mass of benzocaine is 165 g/mol.

Find the molecular formula of benzocaine.

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