stoichiometry with gases

Subject:        Chemistry Lab Question

Hi Holly,
I have a problem to solve for my chemistry lab and am having a brain
fart. We are supposed to determine the atomic mass of an unknown metal.
The question reads:
A sample of 40.0 mg of a metal reacts with HCl according to the
equation in the text to generate 24.4 mL of H2 gas at 20 degrees C. The
gas is collected by the downward displacement of aqueous solution of acid.

The atmospheric pressure is 780 mm Hg. Calculate the atomic mass
of the metal.

So, I started with the "shopping list" of the given information.
I wrote down PV=nRT and manipulated it to get n=PV/RT. I also
calcualted the P to be 1.02632 atm and V to be 0.0244 L of H2. The R is
constant and T was converted to 293K. I solved the equation and came up
with n=0.001041 moles. I then divided the 0.040 g by 0.001041 moles and
came up with 38.4246 g/mole. I don't know what to do next or if I
missed a step in the process or should start over.

If you would like, I could come to your office tomorrow sometime and you could show me or if you could email me back with a hint.

Whichever is easier for you.
Thanks!!!
SR

Dear SR,
Hmmm! It all looks good except you may have gone one step too far in dividing g by moles.

I would want to see the reaction "a metal reacts with HCl according to the
equation in the text"...

In general, metals plus acid make H2 plus the oxidized metal:

M  +  2H+    make      M+         +   H2
But depending on how many electrons the metal loses, the reaction will need to be balanced according to charge on both sides.

I would relate the moles of gas that you calculated to moles of metal in the reaction equation because they may not be the same--look at their coefficients. Hopefully your lab handout gives you some equation?? Then you may have to multiply the number of moles or divide it by some number like 2 or 3. Even so, I am not sure that works (I did some quick calcs) to give you an obvious metal's atomic weight.

The only thing I can think of is to use stoichiometry to relate the number of moles of H2 to moles of metal (they may or may not be the same.) If it really is 38 grams per mole, I guess the closest thing to it is potassium...I am guessing this is a prelab theoretical question, and hope you are not really working with K metal in your lab because it is explosive.

Let me know if that helps.
Good luck!
Holly

Comments

[Holly Phaneuf Erskine]

This comment came from RK, I am reposting it myself as I am moving my posts to a nicer template. Apparently I never addressed RK's question here (yet)

Could flame tests be useful in determining identities of metals in a mixture? If so, what problems might arise? If not, why not?