1. October 25 & 26 2012 Ms. Boon Chemistry
Stoichiometry #5
October 25 &
Ms. Boon Chemistry

2. Objective:
Catalyst
When you hear the term “experimental error” what do you think of? Write at least 3 sentences.
I can use stoichiometry to calculate the theoretical yield and percent yield of a chemical reaction.
Agenda:
Catalyst
Lab Day 2: Copper and Silver Nitrate
Notes: Percent Yield & Experimental Error
Practice: Stoichiometry
Exit Slip
HW: Problems: p. 317 #1-3.
Review: p. 298 #1, 2, 3, 5

3. Lab Day 2 Continue with the lab procedure where you left off.
Be sure to complete Data Table 1 and the Day 1 calculation.
Record observations on the paper you are using for calculations.
Some things to think about when observing:
What colors do you see?
Describe the size of objects. Grab a ruler to make quantitative observations.
What is the solid that forms? Is it copper or silver or something else?
Describe how fast or slow the reaction is happening using a timer or counting seconds or minutes.

4. Lab Day 2 Safety Procedure
Choose 2 students from your table to actually handle the materials for the day 2 procedure.
These students must wear safety goggles and gloves.

5. Notes: Types of Experimental Error
Scientists always report all sources of error that may affect experimental data.
Two types of experimental error:
Human error = making a mistake in the lab
Systematic error = measurements are not accurate because the equipment is not calibrated correctly, is broken, or is not sensitive enough.

6. Quick Practice: Human or Systematic Error?
When swirling the test tube to shake the silver off, a splash of liquid escaped.
human
When pouring out the contents of the test tube, some bits of silver stuck to the test tube and were not part of the final mass measurement.
Human
The triple beam balance said the copper wire had a mass of 1.2 g. A newer electronic scale measured the copper at g.
systematic

7. Notes: What is percent yield?
Percent yield describes the efficiency of a chemical reaction.
It is a comparison between the actual yield and the theoretical yield.
Calculate the percent yield:
actual yield X % = percent yield
theoretical yield

8. Calculate the percent yield: actual yield X 100% = percent yield
theoretical yield
You try 1: The theoretical yield is 200 g. The actual yield is 150 g. What is the percent yield?
You try 2: The theoretical yield is 50 g. The actual yield is 25 g. What is the percent yield?
You try 3: When 34 g ammonia are combined with excess oxygen, the theoretical yield of nitrogen gas is 28 g. The actual yield is 22 g. What is the percent yield of nitrogen gas?
Calculate the percent yield:
actual yield X % = percent yield
theoretical yield
Example: The theoretical yield of silver when you combine 8g copper and 1.7g AgNO3 is 1.08 g Ag. The actual yield is 0.92 g Ag. What is the percent yield?
0.92g Ag X % = 85%
1.08 g Ag

9. Notes: Percent Yield (to practice – p. 319 #6, p. 331 #34)
Example: hydrogen and oxygen form water
2 H2 (g) + O2 (g) → 2 H2O (l)
Identify the limiting reactant and the theoretical yield of water (grams) when 10 g hydrogen react with 32 g oxygen. The actual yield is 27g water.
What is the percent yield?
Step 1: Split the question into two stoichiometry problems.
Step 2: Solve both problems.
Step 3: Compare answers. The smaller product amount is the theoretical yield. The limiting reactant is the reactant that that formed less product.
Step 4: Percent yield – divide actual yield/theoretical x 100%
How many g H2O form from 10 g H2?
How many g H2O form from 32 g O2?

10. Notes: Percent Yield (to practice – p. 319 #6, p. 331 #34)
Example: reaction of ammonia and oxygen
4 NH3 + 3 O2 → 6 H2O + 2 N2
Identify the limiting reactant and the theoretical yield of Nitrogen (grams of N2) when 34 g ammonia (NH3 ) react with 64 g oxygen (O2).
The actual yield is 24 g N2. What is the percent yield?
Step 1: Split the question into two stoichiometry problems.
Step 2: Solve both problems.
Step 3: Compare answers. The smaller product amount is the theoretical yield. The limiting reactant is the reactant that that formed less product.
Step 4: Percent yield – divide actual yield/theoretical x 100%
How many grams of N2 form from 34 g NH3 ?
How many grams of N2 form from 64 g O2?
34 g NH3 / 17 g/mol = 2 mol NH3 x 2 mol N2/4 mol NH3 = 1 mol N2 x 28g/mol = 28 g N2 --- this is smaller. This is the theoretical yield. The smaller product amount came from the calculation with NH3 – that is the limiting reactant.
64 g O2 / 32 g/mol o2 – 2 mol O2 x 2 mol N2/3mol O2 = 4/3 mol N2 x 28 g/mol = 37.3 g N2
Percent yield – actual/theoretical /28 = 0.86 x 100% = 86%

11. Exit Slip 10.25&26 – Standard 3a, 3d, 3e, 3f
Balance the chemical reaction equation.
Li + O2  Li2O
What is the molar mass of Li?
What is the molar mass of Li2O?
What is the molar mass of O2?
What is the mole ratio of Li2O to Li in the balanced equation?
What is the mole ratio of Li2O to O2 in the balanced equation?
How many grams Li2O will be produced from 56 g Li?
How many grams Li2O will be produced from 32 g O2?
Identify the limiting reactant and the theoretical (calculated) yield of silver Li2O in grams, when 56 g Li reacts with 32 g O2.
If the actual yield is 30 g Li2O, what is the percent yield?