Chemistry help, please

[swimmermom3]

I worked the following problem and came up with the right answer, but when I talked it over with ds, I think it might have been a fluke because I didn't take into account the water vapor pressure:

 

5.61 (from Chang's General Chemistry, 5th ed.)

 

A piece of sodium metal undergoes complete reaction with water as follows:

 

2Na(s) + 2 Hsub20(l)  right arrow 2 NaOH (aq) + Hsub2 (g)

 

The hydrogen gas generated is collected over water at 25.0 degrees C. The volume of the gas is 246mL measured at 1.00 atm. Calculate the amount of zinc metal in grams consumed in the reaction. (Vapor pressure of water at 25 degrees C = 0.313 atm.)

[swimmermom3]

Anyone care to try? :tongue_smilie:

[Dicentra]

Just saw this, SM3!  I'll take a shot. :)

 

First thing, though - I'm assuming you meant to say amount of sodium metal, not zinc metal, in the question above otherwise the whole thing makes no sense. ;)

 

I got an answer of 0.318g of sodium metal.  Is that what you and ds got?

 

Before you can use the ideal gas law, you have to use Dalton's Law of Partial Pressures.  When you collect a gas over water, part of the pressure inside the container is due to water vapour so you have to correct for that.  As long as you are measuring your total pressure in the same units as the correction factor you have for water vapour, it's just a simple subtraction.  So for your question:

1.00 atm - 0.313 atm = 0.687 atm

The reason for the subtraction is because when you use the ideal gas law, the "P" you need is the pressure due to the hydrogen gas alone, not the pressure due to hydrogen gas plus water vapour - hence the need to subtract out the pressure due to water vapour.

 

Once you have the pressure of hydrogen gas alone, you can use the ideal gas law to calculate the moles of hydrogen gas.  Make sure that you convert your volume of gas to litres and your temperature to Kelvin.  Also make sure you use the correct value of R for your pressure units - in this case, you should be using 0.0821 L*atm/K*mol.  Rearrange the ideal gas law, plug in the numbers, and you should get 0.00691 mol of hydrogen gas.  Use stoichiometry to determine the moles of Na metal and you should get 0.01382 mol of Na.  Convert to a mass using the atomic mass of Na and you should get 0.318 g.  (Note:  I was playing kind of fast and loose here with sig figs so your answer may be slightly different than mine. :))

 

Does that help, SM3?