Combustion analysis TBR Chemistry

[lychee3]

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p. 34, question 11

Which of the four compounds is least likely to contain oxygen?
A. I
B. II
C. III
D. IV

I don't understand the explanation... says the compound least likely to contain oxygen is the one that produces the most CO2 upon combustion. I thought that you would have to look at both H2O and CO2 because they both contain oxygen.

Also, would reaction between magnesium sulfate + water form a magnesium sulfate hydrate?

 

[BerkReviewTeach]

p. 34, question 11

Which of the four compounds is least likely to contain oxygen?
A. I
B. II
C. III
D. IV

I don't understand the explanation... says the compound least likely to contain oxygen is the one that produces the most CO2 upon combustion. I thought that you would have to look at both H2O and CO2 because they both contain oxygen.

Also, would reaction between magnesium sulfate + water form a magnesium sulfate hydrate?

If you consider combustion analysis of an organic molecule made up of C, H, and O, then you can safely say that CO2 correlates to C and H2O correlates to H. Because you are burning the compound, the O can come from anywhere.

They have a fixed mass of compound in each sample, so if they all contained the same mass percent of carbon, then they'd all generate the same mass of CO2. Likewise, if they all contained the same mass percent of hydrogen, then they'd all generate the same mass of CO2.

What the explanation is saying is that as you generate more CO2 product, you must have a greater mass percent of carbon in the compound. As you generate more H2O, you must have a greater mass percent of hydrogen in the compound. The greater the mass percent of carbon, the less mass percent of oxygen you can have. The same thing holds true for the mass percent of hydrogen, but because it's so light compared to oxygen, it doesn't tell us as much. Basically what the explanation means is:

• if CO2 formed goes up then mass% of C goes up
  if mass% of C goes up then mass% of O goes down

Most CO2 correlates with the compound that has the least O in its formula (by mass percent).