Question

Which statements regarding combustion analysis to determine percent composition are not correct?
      I. The mass of oxygen in the sample compound can be determined from the mass of carbon dioxide that is produced.
      II. If a compound contains an element other than carbon and hydrogen, combustion analysis cannot be used to determine its empirical formula.
      III. Combustion analysis data alone is not sufficient to determine the molecular formula with certainty.
IV. If some CO is produced rather than all CO₂, then the empirical formula that is calculated will be too high in carbon.

	a.	I, II, and IV are not correct.
	b.	II and III are not correct.
	c.	I, II, and III are not correct.
	d.	I and II are not correct.
	e.	II, III, and IV are not correct.

Answer 1

To answer this quest it is important to understan the working of a combustion analyser

Any compound can be oxidised completely in the presence of oxygen at high temeperature. This process is what a combustion analyzer employes. This type of an analyzer is used to estimate the amounts of carbon, hydrogen, Nitrogen and in some cases oxygen or sulphur.

The way the analysis is done is carbon is completely oxidised to CO2, Hydrogen to H2O, Nitrogen can be analysed as N2 or its oxides depending on the technique.

Now the amount of CO2 formed is used to detremine the amount of carbon present in the sample not the amount of oxygen.

If a compound contains other elements other than carbon and hydrogen like nitrogen, sulphur or oxygen we can still determine their empirical formula using combustion analysis.

Ofcourse for many compounds Combustion analysis data alone is not sufficient to determine the molecular formula with certainty. We will need other techniques to determine formula with absolute certainity for example an organic ester and another organic compound having both an ether and carbonyl will have similar number of carbon and oxygen units.

As mentioned earlier the amount o carbon in an organic compound is determined as CO2. So if some CO is produced rather than all CO₂, then the empirical formula that is calculated will become lower as we are estimating a lower amount of CO2.

So from the explainations above we can conclude that

a. I, II, and IV are not correct.