Question

One of the ingredients in the Native American stomachache remedy derived from common chokecherry is caffeic acid (molar mass 180.16 g/mol).
Combustion of 1.00 ×10^2 mg of caffeic acid yields 220 mg of CO2 and 40.3 mg of H2O.
What is the empirical formula of this compound? List the elements starting with carbon, followed by hydrogen, then oxygen.

Note:
You are given the mass of CO2 and H2O formed by combustion analysis of caffeic acid. You are asked to determine the empirical formula.

Analyze
You should start by finding the mass of C and H in the original compound from the mass data given. How would you use the 220 mg of CO2 to solve for the moles of C in the original sample?

Answer 1

We can use the mass of CO2 produced during the combustion of caffeic acid to determine the moles of carbon in the original sample.

The balanced chemical equation for the combustion of caffeic acid can be written as:

C9H8O4 + aO2 -> bCO2 + cH2O

From the given data, we know that 1.00 × 10^2 mg of caffeic acid yields 220 mg of CO2.

To find the moles of carbon in the original sample, we need to use the molar mass of CO2.

First, we need to convert the mass of CO2 to moles using its molar mass:

molar mass of CO2 = 12.01 g/mol (for C) + 2(16.00 g/mol) = 44.01 g/mol

Now we can calculate the moles of carbon:

moles of CO2 = mass of CO2 / molar mass of CO2

= 220 mg / 44.01 g/mol

= 0.005 moles

Since the coefficient in front of CO2 in the balanced chemical equation is equal to the moles of carbon in the original sample, this allows us to know that the number of moles of carbon in the compound is also 0.005 moles.

Now we need to determine the moles of hydrogen in the compound using the mass of water produced during the combustion. This will allow us to calculate the ratio of moles of hydrogen to carbon, which will lead us to the empirical formula of the compound.