Question M.19 (Sixth Edition)

[Steve Magana 2I]

Question: A stimulant in coffee and tea is caffeine, a substance of molar mass 194 g.mol^-1. When 0.376 g of caffeine was burned, 0.682 g of carbon dioxide, 0.174 g of water, and 0.110 g of nitrogen were formed. Determine the empirical and molecular formulas of caffeine, and write the equation for its combustion.

I was specifically wondering how we start this problem, I feel as though the 194 g.mol^-1 is what's throwing me off. Thank you!

[Raphael_SanAndres3C]

To start, you have to convert CO2, H2O, and N2 into mols using their molar mass and the grams given to you for each substance (.682g for CO2, .174g for H2O, and .110g for N2). The 194 g.mol^-1 for caffeine isn't important until later when you need to calculate the molecular formula from the empirical formula.

[Brian Chhoy 4I]

This is a combustion analysis question. I would start the question to trying to find the exact masses of carbon, hydrogen, nitrogen, and possibly oxygen too. Take the mass of each product and convert it to mass to the exact mass of that element produced. For example, with the carbon dioxide, divide by its molar mass to get the moles of carbon dioxide produced. Then take the moles of carbon dioxide produced and use the mole ratio between carbon and carbon dioxide (1:1) to get moles of carbon produced, which you then multiply by the molar mass of carbon to get the mass of carbon produced. Continue to do this to the other elements, besides oxygen, to gain the mass of each element so you can follow the steps to find the empirical formula. To find the mass of oxygen, you first have to add up the masses that you calculated from the other elements and subtract that amount from the mass of caffeine burned to get the mass of oxygen from caffeine. Follow to steps to find the formula, such as convert to moles and then dividing by the least number to get the formula. Then use the molar mass to find the molecular formula.