Homework Help: Chemistry

Posted by Louis on Monday, September 5, 2011 at 1:03am.

Need help solving this problem.

A 0.188g sample of unknown metal, X (s), produced 71.4ml of hydrogen gas when reacted with HCl according to the equation:

X(s)+2HCl(aq)--> XCl2(aq)+H2(g)

The gas was collected over water at 23 degrees C. The levels of water inside and outside the gas collecting tube are identical. The vapor pressure of water at 23 degrees C is 21.1mm Hg and the atmospheric pressure is 752 mm Hg. Calculate the molar mass of the unknown metal, X. (R=0.0821Latm/molK)

I got 1.52 x 10^-4 g/mol as my answer. Professor told me it was wrong... Didn't give me the correct answer and told me to keep trying... No assiatance whatsoever. I have been trying all weekend. What am I doing wrong?

Chemistry - DrBob222, Monday, September 5, 2011 at 2:12am
You must find the number of moles in the 0.188 g sample.
How many moles H2 were collected.
PV = nRT
P = (752-21.1)/760 which may be one place you are going wrong. This will be the pressure of the dry hydrogen gas in atm.
V = 71.4 mL (convert to L)
n = solve for this
R = 0.08206
T = 23 C converted to K.
Then look at the equation. moles X will be 1/2 moles H2.
Finally, n = grams/molar mass. YOu know n and grams, solve for molar (atomic) mass. My best guess is Cs

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