Name: ______Date: ______Period: ______Page: ______

PERCENT YIELD OF A SALT

The productivity of a chemical reaction is measured in terms of product yield just as the productivity of a farm is measured in crop yield. Because experimental/growing conditions will vary, the actual yield of product/crop often differs from the expected yield of product/crop. One can calculate the % yield as a means of comparing the actual (experimental) yield and expected (theoretical) yield by using the following equation:

% yield = experimental yield x 100

theoretical yield

In reality the percent yield is usually less than 100% because most chemical reactions do not go to completion, reactants may contain impurities that may cause competing side reactions and yield unwanted products, and/or product is lost due to reagent transfer and filtration (splattering will also result in product loss but this is faulty technique).

In this activity, you will become an industrial chemist. You will experimentally determine the mass of a product in a chemical reaction and then compare this mass to the theoretical mass you would predict based on the chemical equation. Then you will assess the reliability of the data by performing an error analysis.

OBJECTIVES

When you have completed this activity, you will be able to:

  1. Demonstrate a procedure and collect data to determine the experimental yield of a reaction product.
  2. Differentiate between experimental yield and theoretical yield.
  3. Compute the theoretical yield of a reaction product using a balanced equation.
  4. Calculate the percent yield of a salt using experimental data.
  5. Perform an error analysis.

MATERIALS

goggles iron ring balance, sensitive to at least 0.01 g

evaporating dish watch glass sodium bicarbonate (NaHCO3)

crucible tongs ring stand 3.0 M HCl

burner wire gauze dropper

PROCEDURE

  1. Clean and dry an evaporating dish and watch glass. To dry the evaporating dish and watch glass, use tongs to hold them in the burner flame for about 20 seconds, allow to cool, and determine the mass of the evaporating dish and watch glass cover. Record the mass measurement in the Data Table in entry #1. Repeat this heating, cooling, and mass measurement process until the mass is constant. Constant mass is attained when the two masses agree within + 0.01g. Record the mass measurements in the Data Table in entry #2 and in entry #3.
  2. With the dry evaporating dish and watch glass balanced, hit zero, and add approximately 2 grams of solid NaHCO3. Record the actual mass of NaHCO3 solid in the data table in entry #4.
  1. Cover the evaporating dish with the watch glass, curved side down. Using a dropper, add 3.0 M HCl to the NaHCO3. Let the HCl flow down the lip of the dish. Gently swirl the evaporating dish to mix its contents as the acid is added. Continue adding acid by the dropper-full while gently swirling the evaporating dish until all of the solid dissolves and there is no further effervescence upon the subsequent addition of acid.
  1. Place the evaporating dish and watch glass cover on a wire gauze supported by a ring stand. Heat the contents under low heat (flame does not have an inner blue cone) until a solid begins to form. Hold the base of the burner, and move it back and forth under the dish. Continue evaporating to dryness. When completely dry, stop heating, and allow the dish to cool. When cool enough to transport with your hand, place on the balance and measure the mass of the dish, contents, and watch glass cover. Record the combined mass in the Data Table in entry #5.
  1. Repeat the heating, cooling and mass measurement process until constant mass is achieved. Record the combined mass after each measurement in the Data Table in entry #6 (and #7).
  1. Determine the mass of the residue (experimental yield) and record in the Data Table in entry #8.

DATA TABLE

1. Mass of evaporating dish and cover (first heating) ______g

2. Mass of evaporating dish and cover (second heating) ______g

3. Constant mass of evaporating dish and cover ______g

4. Mass of NaHCO3 ______g

5. Mass of evaporating dish, cover, and residue (first heating) ______g

6. Mass of evaporating dish, cover, and residue (second heating) ______g

7. Constant mass of evaporating dish, cover, and residue ______g

8. Mass of residue (experimental yield) ______g

Name: ______

ANALYSIS

1.  Write a balanced equation for this reaction if the reactants yield a salt, a gas, and water as products.

______

2.  Calculate the number of moles of NaHCO3 used in the experiment.

Moles of NaHCO3 ______

3.  Use the balanced equation and the moles of NaHCO3 computed in #2 to calculate the mass of solid product, NaCl, that should be produced (theoretical yield).

Theoretical Yield of NaCl ______

4.  Calculate the % yield of salt.

Percent Yield ______

5.  Perform an error analysis.

a)  Calculate the % error using the theoretical yield from analysis question #3 and your experimental yield from the data table.

Percent Error ______

b)  List 3 sources of error. Do not state “human error” or “error with calculations.”

a.  ______

b.  ______

c.  ______

PRE-LAB: PERCENT YIELD OF A SALT

Read the introduction and the procedure of the lab activity, highlighting key information as you read. Answer the following questions.

1)  Differentiate between the terms experimental yield and theoretical yield.

2)  How is a % yield calculation different from a % error calculation? Use the formulas for each calculation in your answer.

3)  In industry, chemists are often hired to modify experimental conditions to maximize the % yield of a desired product. Why would an industrial corporation be interested in maximizing % yield?

4)  In an attempt to save time, one of the students in your group suggested that you eliminate steps 1 and 5 (the repeated heating, cooling, and mass measurement steps). What effect might this have on:

  1. the experimental yield?
  1. the theoretical yield?
  1. the % yield?