Chem Class- November 5, 2009

We went over more of our unit 2 homework questions and then we got started on a new topic:

 Density and Moles.

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Density----->Mass per unit volume

D = m/v

m=Dv

v=m/D

Density of a gas at STP- 1 mole of gas

Dstp = m/v = Molar Mass g/mol/22.4l/mol

Ex: Calculate the density of CO2 at STP
Density of CO2 (g) = 44.0 g/22.4 L =1.96 g/L

Density of air is approximately 1.5 g/L.

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The following chart shows the conversions between mass, particles and volume.

Summary of the conversions we've learned so far and their conversion factors:

Moles<----->Mass: (molar mass) g/ 1 mol or 1 mol/(molar mass) g

Moles<----->Volume (gases at STP) 22.4 L/1 mol or 1 mol/22.4 L

Moles<-----> Number of Particles (Atoms or Molecules):

6.02 x 10 to the power of 23 particles/ 1 mol or 1 mol/6.02 x 10 to the power of 23 partilces

Molecules<----->Atoms: # of atoms/ 1 molecule or 1 molecule/# of atoms

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More Examples:

1) The density of CCl4(l) is 1.59 g/mL. How many molecules of CCl4 are there in 2.59 L of CCL4?

# molecules = 2.50 L x 1.59 g/ 10 to the power of negative 3 L x 1 mol/154.0 g x 6.02 x 10 the power of 23 molecules/ 1 mol = 1.55 x 10 to the power of 25 molecules

2) What is the mass of 250.0 mL of C3H8?

Mass = 0.2500 L x 1 mol/22.4 L x 42.0 g/1 mol = 0.469 g

3)How many moles are there in 6.00 L of NO3F(g) at STP?

# of moles= 6.00 L x 1 mol/22.4 L = 0.268 mol

4)What is the volume occupied by 10.0 g of H2S(g)?

Volume = 10.0 g x 1mol/34.1 g x 22.4 L/1 mol = 6.57 L

Ahaha! Here's a song that kind of sums up a lot of what we've learned for this unit so far: