Gas Stoichiometry

Hey Everyone! Today the lesson we learned was on Gas Stoichiometry. In class we went over our 

Ideal Gas Law WKSHT that was due today (answers posted on moodle). We then spent the rest of the period learning Gas stoich.

Stoich using PV = nRT

Mr Lieberman pulled out the mole cannon today and used it as our demo for today's lesson. Here's the video. (Don't know why you guys are so frightened?)

1. So in order to find the pressure of the gas in the tube you have to make and balance a chemical equation. So this is it for the mixture of the solid and gases in the tube.

CaC_2  (s)  + 2H_2 O ------>  C_2 H_2 (g) + Ca(OH)_2  (aq)    (sorry, underscore means subscript)

2. Next, you find the moles of gas formed from the reaction. Just a reminder, CaC_2 is the limiting reactant while the C_2 H_2 is the excess reactant.

   .37 g CaC_2              1 mole CaC_2               1 mole C_2 H_2

_____________  x   ________________  x  _________________  =     .006 moles.

            1                                64g                         1 mole CaC_2

3. Next, you have to find the pressure created by the gas. In order to do this, let's use PV=nRT!!

P=?      use PV=nRT     or    P= nRT/V

V=  .87L  (use the volume of cylinder that Mr. Lieb gave us)

T= 25 degrees Celsius + 273  =  298K  (Has to be in Kelvin)  (25 degrees is room temperature)

Moles=  .006 Moles   (work from above)

R=  .082  atm/moles K  (constant for gas)

Now that we have everything we need, we set up the problem!!!

       (.006) x (.0821) x (298K)  

P=  _____________________

      .87L

So in conlcusion, the pressure of the gas in the tube would be 0.17!

You could also solve stoich by using the Molar volume formula with STP. A.K.A. Standard Temperature Pressure

At STP, 1 mole of any kind of gas = 22.4L    (Molar Volume)

Sample Problem: Sodium azide is used in airbags to cause them to inflate on impact. What volume of nitrogen gas is formed from the decomposition of 1.00g of sodium oxide (NaN_3)? Assume STP conditions.

So first is the balanced chemical equation.

3 NaN_3 ------->  4N_2    +    Na_3  N

So we know this much....

T=273K

P=1 atm.

Let's use the formula

1.00g NaN_3        1 mole NaN_3            4 moles N_2               22.4L

____________ x ______________ x  ______________ x  ____________ = 0.46 

          1                           65g                  3 moles NaN_3         1mole N_2

See, you didn't have to use PV=nRT when you know you can sue STP! Time saver!!!

So that's pretty much what we learned in class today!

HMWK: Study for Quiz Tomorrow     (Good Luck on it!)

               Gas Stoich #1 WKSHT

               Ideal V.S. Combined WKSHT

The next scribe is Danielle Sindelar!!!  (Sorry to other people who wanted to be scribe)

Have a Great Thanksgiving Break!!!!!