chem I questions?

Question

5. Determine the number of moles of Krypton contained in a 3.25 liter gas tank at 5.80 atm and 25.5 °C. If the gas is Oxygen instead of Krypton; will the answer be the same? Why or why not?

1.
If the outside of the flask is not dried after vaporizing the liquid, will the unknown's calculated molecular weight is too high or too low? Explain?
2.
Suppose the atmospheric pressure is assumed to be 1 atmosphere instead of its actual value. How wills this error affects the molecular weight of the unknown? Explain.
3.
If the vapor's volume is assumed to be 125 mL instead of the measured volume, what is the percent error of the unknown's calculated molecular weight? Show your work.
4.
If the entire unknown does not vaporize in the 125 mL Erlenmeyer flask, will the reported molecular weight be too high or low? Explain.

Answer 1

Data required.

Answer 2

PV = nRT

Since you would be solving for moles using the same P, V, T and R for Krypton or Oxygen, the identity of the gas doesn't matter. That's the cool thing about gases, the identity does NOT matter. You would get the same number of moles, regardless of which gas was in the flask.

You are solving for moles, because you know how many grams of substance you put in the flask.

n = PV/RT

If you did not dry the flask completely, then your grams would appear more than they actually are: mm = grams/moles

Since grams is in the numerator, your mm would be too high

Since P is in the numerator, your number of moles that you calculate would be high or low depending on what the actual pressure was. If the actual pressure was lower than 1 atm, then your moles would be too high, since mm = grams/moles, moles big number means mm = smaller number. If the atmospheric pressure was higher than 1 atm, your number would be a smaller number of moles and so your mm would be higher than it was supposed to be (I can't answer this one directly since you didn't tell me what your class atm P reading was!)

Again, n = PV/RT

V is in the numerator. The EXACT same arguement as above given for pressure. Again, since you didn't tell me if your actual volume was higher or lower than 125 the dual arguements above answer this.

If the sample does not entirely vaporize, then you will be assuming that you have a gas when you don't. When you go to condense your gas back down again, it wasn't all vaporized so your reading will be low, because it wasn't all in the gas phase.

Here's a thought for #2 and #3 - plug in numbers!!

n = PV/RT plug in what you were supposed to get. mm = grams/moles so solve for the "correct" mm.

Use the P or the V given in each problem and plug into n = PV/RT and solve for mm again!

% difference = absolute value(your value - the correct value)/correct value all times 100