Calculating Final Pressure in a Sublimation and Pressure Homework Problem

[Qube]

Homework Statement

When fluorine and solid iodine are heated at 550. K, the iodine completely sublimes and gaseous iodine heptafluoride forms. If 350. torr of fluorine gas and 2.50 g of solid iodine are put into a 2.50 L container at 250. K and the container is heated to 550. K, what is the final pressure in the flask?

Homework Equations

5F2 (g) + I2 (s) --> 2IF5(g)

The Attempt at a Solution

We have 350/760 atm of fluorine gas. Using the ideal gas law, P = nRT/V, we find taht we have 0.056 moles of F.

We also have 0.009 moles of solid iodine (2.5 grams / twice the molecular mass of elemental iodine).

This yields 0.0197 moles of product. Iodine is limiting and fluorine is in excess. The final pressure, by Dalton's Law of Partial Pressures, is the pressure of the product and the pressure of the remaining reactants.

We have 0.0197 * (5/2) moles of fluorine used. This means we have 0.056 minus 0.0475 moles of fluorine remaining. This means there are 0.085 moles of fluorine remaining. The pressure of this remaining fluorine is:

P = nRT/V = 0.085(0.0821)(550)/2.5 = 0.15 atm.

The pressure of the product is also nRT/V = 0.0197 * 0.0821 * 550 / 2.5 = 0.355 atm.

Summing these two pressures, we get 0.50 atm of pressure. However, this isn't an answer choice. The closest choice is 0.534 atm, leading me to believe there is an error in my calculations, and not a rounding one at that. Is there any flaw in my line of reasoning?

 

[Borek]

What is the formula of the iodine heptafluoride?

 

[Qube]

Oh snap. Hept ... that's prefix for 7. I confused that with pentafluoride. Thanks for the catch!

 

[Qube]

Okay, new balanced equation:

7F2 + I2 --> 2IF7.

New process.

We have to convert everything to atmospheres of pressure at 550 K to solve this problem. This is the most convenient way. My method doesn't work because it doesn't account for the temperature change and how that might affect the Fluorine in terms of pressure. I can't use pressure of F at the initial temperature and arrive at an amount of Fluorine; it'll change when the temperature changes.

1) Find the atmospheres of I2 at 550 K. P = nRT/V. P = 0.177 atm.

2) Find the atmospheres of F2 at 550 K. P / T = P / T. Convert torr to atm (350/760) and temperatures are in Kelvins (250 and 550 K). Solve for the new pressure of F2.

3) Now we can set up this classic problem (classic for my teacher; he often gives us atmospheres of pressure and has us to a stoichiometry problem; this just takes it a step further by withholding the proper pressure at the proper temperature). Use atmospheres of pressure for each reactant and find which one will result in the least amount (pressure) of product. Total pressure = pressure of product + pressure of left over reactants.

 

[DeeNos]

Your line of reasoning is correct, and the error likely lies in a rounding error in one of your calculations. It is always important to double check your calculations and ensure that all units are consistent throughout the problem. Additionally, make sure to round to the appropriate number of significant figures in your final answer. In this case, the final answer should be rounded to three significant figures, giving a final pressure of 0.534 atm, which is the closest answer choice. It is also good practice to label all of your calculations and clearly show your work, as this can help identify any errors or mistakes. Overall, your approach to the problem is correct and with careful checking and rounding, you should be able to arrive at the correct answer.