Chemistry Hard! Gas laws?

Question

Mg (s) + 2 H3O+ (aq) --------> Mg2+ (aq) + H2 (g) + 2 H2O (l)

1) If the vapor pressure of water at 20.0 oC is 17.535 torr, and the atmospheric pressure measured by a barometer was 757.3 torr, what is the partial pressure of H2 in the gas mixture in a buret?

2) If the volume of H2 gas collected was 47.92 ml at the temperature and pressure given in problem #1, then
calculate the moles of H2 gas produced (be sure to convert P, T and V to the same units found in R!!).

3) Now calculate the mass of Mg metal reacted using stoichiometry and the balanced reaction above.

4) Determine the mass of Mg reacted using the unit equation approach involving converting the volume of H2

Volume H2 at STP =
Mass Mg reacted =

I need an answer key to check with mine, thanks in advance!
gas to STP conditions,

Answer 1

1) Subtract water v.p. from total pressure to find H2 P



2) Use P1V1/T1 = P2V2/T2 to fond vol H2 at STP (P atm; L liters; T K)



OR use PV = nRT (I don't knw why the Q seems to want you to do both)



3) From n(H2), infer n(Mg)



4) STP Vol H2(L) x 1 mol/22.4 L at STP x 1 mole Mg/1 mole H2 [why is that true?] x 24.3 g Mg/1 mole Mg = ?? g Mg



I'm doing it this way because the Q asked me to. Given the choice, I always use PV = nRT to find number of moles



I have NEVER seen STP except in the gas laws classroom

Answer 2

If you collect a non-soluble gas over water, you need to subtract out the vapor pressure of the water, since it will saturate the collected gas.

1. H2 pressure= measured pressure-VP of water at 20C.

2. You don't need us to do PV=nRT for you. You found P above, you are given V, you know T, you want to find n. Your book will give you an R for torr-mL/mole-oK or torr-L/mole-oK.

3. The stoichiometry tells you that for each mole of H2 generated, one g-atom of Mg is consumed. So multiply the moles from problem 2 by the g-wt of Mg (about 25).

4. I wouldn't do all this mathematical torture (I could think of a more sordid term, but this is a family service). Why do something a second time when you have one way to do it correct. But if you need to, you have n and you know the conversion for 1 mole, so you have the proportion: n moles/1 mole = X ml / 22414 mL

Answer 3

This is something of an odd question. At first, it seems like you would need the PV=nRT equation, but there are no variables in that formula that you would be solving for. Instead, this is a limiting reagent (also known as limiting reactant) problem. Since the moles produced by a reaction are not affected by pressure, temperature, etc (unless it is an equilibrium problem, which your single-headed arrow tells me this is not), you simply use a mole-to-mole conversion. So we get: 5g S8 x (1 mol S8/256.8g S8) x (8 mol SO2/1mol S8) x (64.1g SO2/1 mol SO2) = 9.9844g SO2 This is the only way I can think of to do this problem. Chemistry is particularly bad with giving you more information than you need for the problem in order to trip you up. I hope this helped! Note: I am confident in my answer, but you may want to double-check my math to make sure I didn't punch something into the calculator wrong.