Re: plz someone help with this problem relating to chemical reaction


"Marcus Fox" <please-reply-via-newsgroup-th@xxxxxxxxxxxxxxxx> wrote in
message news:cPidnf-q1sM2R8HbnZ2dnUVZ8qKvnZ2d@xxxxxxxxx

"The_Man" <me_so_horneeeee@xxxxxxxxx> wrote in message
news:1180478889.702508.233990@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On May 28, 4:36 pm, reactionhelp <nisarahme...@xxxxxxxxx> wrote:
What mass of magnesium, in reaction with an excess of hydrochloric
acid, is needed to produce 128 ml of hydrogen (h2) gas? The h2 is
collected over water at a temperature of 25c and a total pressure of
762 mmhg.

HInts (for an obvious homework problem)

1) Write (and balance) the reaction of Mg with HCl to give H2.
2) Look up the partial pressure of water at 25C. The pressure of H2 is
the total pressure (762 Torr) MINUS the partial pressure of water
(Dalton's Law of partial pressures)
3) Find the number of moles of H2 by using the ideal gas law (PV=nRT).
Make sure you use the PARTIAL pressure of H2, and NOT the total
pressure
4) Use the number of moles of H2 (from the gas law) to find the number
of moles of Mg (from step 1).
5) Use the atomic weight of Mg (look this up in periodic table) to
find the mass of Mg needed. Don't worry about the amount of HCl, since
the problem tells you it is excess.

Can you not just calculate how many moles of H2 there are in 128ml of gas,
as one mole of gas occupies a certain volume at STP, and then use your
balanced equation to work out the mass of magnesium?

OK, my bad, it's at 25 C.

Marcus


.

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