<p>I have a problem with test 2 question 11 and wonder if anyone can explain why the answer is correct.</p>
<p>Exhaust is collected from the tailpipe of a car and then 1 mL is put in a syringe to be tested for percent CO (carbon monoxide) in the exhaust. Question 11 asks how the results would be affected if the syringe contents were contaminated with CO-free air (a mixture of different gases not including CO). If I understand the question correctly, the amount of CO does not change, but other gases which were not originally there end up in the syringe. The answer key says the sample would have a lower percentage of CO than before, explaining that the amount of CO is the same (as I assumed), but because there are other gases, the percentage of CO would decrease.</p>
<p>However, the volume of gas in the syringe stays constant (1 mL), so because the amount of CO is the same, the percent CO by volume should stay the same (because of KMT, the particles of the other gases do not affect the volume, etc. of the CO in the sample), so I think the answer should be that the results would not be affected.</p>
<p>Badly phrased question. They should specify what kind of percent they have.</p>
<p>And the volume will change. Gas is not compressible (1 mol/22.4 liters). More particles = more volume. So the percent CO by volume goes down.</p>
<p>Sorry, I thought I had mentioned it said percent by volume, but reading over the post, I didn't.</p>
<p>However, a basic property of gas is that it is highly compressible. Gases do not have definite volume. The relationship 1 mol = 22.414 L only works at standard temperature and pressure. However, the volume has to be fixed in the syringe. Since the volume is fixed adding moles of gas would just increase the pressure in the syringe, but not change the percent by volume of CO.</p>
<p>It doesn't say the pressure isn't fixed. It doesn't say the volume is constant either. That's what makes it a bad question - it leaves too many variables out. In a situation like this either pressure or volume can change with the addition of extra moles. Obviously, if pressure is constant, the percentage decreases, and if the volume is constant, the percentage stays the same.</p>
<p>It's not possible for the volume of CO to be constant, the volume of the syringe to stay at 1 ml, AND to have other gases present (unless the pressure in the syringe goes up)</p>
<p>If the pressure increases, the percent CO by volume will go down. For instance, if you added say 1 ml of another gas to the same syringe at constant volume, the percentage by volume of CO would be cut in half and the pressure would double.</p>
<p>I think the question is intended to mean that the sample of 1ml at 1 atm of pressure is contaminated when drawn, so that the gas has a smaller percentage of CO just because its contaminated with other gasses</p>