Fluorides of the relatively unreactive noble gas xenon can be formed by direct reaction of the elements at high pressure and temperature. Depending on the temperature and the reactant amounts the product mixture may produce XeF2, XeF4, and XeF6. Under conditions that produce only XeF4 and XeF6, 1.85 E-4 mol of Xe reacted with 5.00 E-4 mol of F2, and 9.00 E-6 mol of Xe was found to be in excess. What are the mass percents of XeF4 and XeF6 in the final product

Mass percent XeF₄: 13.78%

Mass percent XeF₆: 86.22%

Explanation:

Moles of Xe are equal to moles of XeF₄ + Moles XeF₆:

Moles Xe:

1.85x10⁻⁴mol - 9.00x10⁻⁶ mol = 1.76x10⁻⁴ moles Xe reacted:

1.76x10⁻⁴ moles = Moles of XeF₄ + Moles XeF₆ (1)

Moles of F₂ = 1/2 Moles XeF₄ + 1/3 Moles XeF₆

5x10⁻⁴ = 2 Moles XeF₄ + 3 Moles XeF₆ (2)

Replacing (1) in (2):

5x10⁻⁴ = 2 Moles XeF₄ + 3 Moles XeF₆

5x10⁻⁴ = 2 (1.76x10⁻⁴ moles - 1 Mol XeF₆) + 3 Moles XeF₆

5x10⁻⁴ = 3.52x10⁻⁴ - 2 Mol XeF₆ + 3 XeF₆

1.48x10⁻⁴ = Mol XeF₆

And moles XeF₄:

Moles XeF₄ = 1.76x10⁻⁴ moles - 1.48x10⁻⁴ moles = 2.8x10⁻⁵ moles

In grams (Molar mass XeF₄: 207.28g/mol: XeF₆: 245.28g/mol):

Mass XeF₄: 2.8x10⁻⁵ mol * (207.28g / mol) = 5.8x10⁻³g

Mass XeF₆: 1.48x10⁻⁴ mol * (245.28g / mol) = 0.0363g

Mass percent XeF₄:

5.8x10⁻³g / (5.8x10⁻³g + 0.0363g) * 100 = 13.78%

Mass Percent XeF₆:

0.0363g / (5.8x10⁻³g + 0.0363g) * 100 = 86.22%

1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.