A research chemist obtains an unknown liquid that contains only carbon
CHEMISTRY · Multi-part · A Level · Chemistry · Amount of substance · Amount of substance
Problem
A research chemist obtains an unknown liquid that contains only carbon, hydrogen, and oxygen. To characterize the substance, she completely burns a carefully measured sample of the liquid in excess oxygen and collects the gaseous products. She records the following raw data for a single trial:
• mass of liquid sample burned
• mass of carbon dioxide produced
• mass of water produced
The chemist asserts that these three measurements, in combination with the law of conservation of mass, are sufficient to deduce the empirical formula of the liquid.
Answer the following questions about her reasoning.
(a) Describe how each of the three measured masses listed above can be translated into the number of moles of an element originally present in the liquid sample.
(b) Explain why a balanced chemical equation for the combustion must be written before the chemist can convert the measured product masses into the desired mole ratios of the elements in the unknown liquid.
(c) The chemist does not measure the mass of molecular oxygen that reacts. Justify why ignoring the oxygen supplied by the oxidant does not prevent correct determination of the empirical formula of the liquid.
(d) Suppose minor experimental error causes the calculated C:H:O mole ratios to be 1.00 : 1.98 : 1.01. Explain how the chemist should interpret these values when reporting the empirical formula.