Chemistry 1LC Practical Practice Test

In combustion analysis, the goal is to determine the empirical formula by finding the amounts of carbon, hydrogen, and oxygen in the original sample. The key idea is that burning the sample converts C to CO2 and H to H2O, so measuring those products lets you back-calculate the amounts of C and H. Converting the masses of CO2 and H2O to moles gives you the mole amounts of C and H directly (one mole of CO2 contains one mole of C, and one mole of H2O contains two moles of H). To get oxygen, you balance by difference: subtract the mass of C and H from the original sample mass, and convert that remaining mass to moles of O. Finally, you simplify the three mole values (C, H, O) to the smallest whole-number ratio to obtain the empirical formula. This is exactly the approach described: collect CO2 and H2O, convert to moles of C and H, determine the simplest ratio, and account for oxygen by difference if needed. Using only CO2 or only H2O omits essential information, and assuming all remaining mass is oxygen ignores the presence of other elements, so those alternatives don’t yield the full empirical formula.