March 7th

Today we'll be doing two NEW but OLD types of stoichiometry calculations! They're OLD because they're done using the concepts we learned before, but there's NEW stuff to be learned as well; we're just introducing stoichiometry to them.

So you can think of these concepts as VINTAGE (get it?) :)

Vintage clothing: always great for a night out with friends!

First, let's do stoichiometry calculations with

MOLARITY

Now, as you may recall, the formula for calculating molarity is M = mol / L, where M is molarity, mol is the number of moles, and L is the volume.

This formula could also be manipulated into mol = M x L or L = mol / M.

Question: Methane gas reacts with oxygen to form carbon dioxide and water. If 0.45 mols of methane reacted to form 1200 mL of water, what is the molarity of the water?

To do this problem, go along the following steps.

1. Write out the word equation.

C

H4

+

O2

--->   C

O2

+

H2

O

2. Balance it!

1C

H4

+

2O2

--->

1C

O2

+

2H2

O

3. Now that you have your mole ratio, know what you HAVE and where you want to GET TO. You HAVE the VOLUME (1200 mL) of the water, you want to calculate the MOLES of the water in order to find the MOLARITY of the water.

0.45 mol CH₄ x 2 mol H₂O/ 1 mol CH₄

= 0.9 mol H₂O with sig figs = 0.90 mol H₂O

4. Now use the molarity formula to figure out the molarity of the water. Since the volume given to you is in mL, divide by 1000 to get a litre quantity.

M = 0.90 mol H₂O / 1.2 L H₂O

= 0.75 M H₂O

And there you have it! Calculating stoichiometry with MOLARITY!

Now let's do stoichiometry calculations with

STP

Now as you may recall, the conversion factor for moles to STP is 1 mol / 22.4 L.

Question: Beryllium fluoride reacts with chlorine gas to form beryllium chloride and fluorine gas. If 8.20L of fluoride gas is produced, how many grams of beryllium fluoride was needed?

To do this problem, go along the following steps.

1. Write out the equation.

BeF₂ + Cl₂ ---> BeCl₂ + F₂

2. Balance it to obtain the mole ratios.

1BeF₂ + 1Cl₂ ---> 1BeCl₂ + 1F₂

3. Now, using the conversion factor for STP, figure out the steps used to get where you want to go.

8.20L F₂   x 1 mole F₂/ 22.4L F₂  x 1 mol BeF₂  / 1 mol F₂  x 47g BeF₂ / 1 mol BeF₂

= 17.2 g BeF₂  (watch your sig figs!)

There! That wasn't so hard, was it? :D

Here's a helpful little PP presentation that will clear up any confusion about stoichiometry conversions using mole ratios, STP, and molarity!

Stoichiometry - Mole Calculations

UNTIL NEXT TIME!