Stoichiometry of a precipitation reaction labpaq answers

Stoichiometry of a Precipitation Reaction Hands-On Labs, Inc. Version 42-0201-00-02

Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.

Experiment Summary:

You will learn about precipitation reactions. You will learn how to use stoichiometry to predict the quantities of reactants necessary to produce the maximum amount of precipitated product. Finally, you will calculate percent yield from a precipitation reaction and determine conservation of mass.

EXPERIMENT

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Learning Objectives Upon completion of this laboratory, you will be able to:

● Identify and define the parts of a chemical reaction, including the reactants and products.

● Identify the defining characteristics of a precipitation reaction.

● Define the term stoichiometry, and discuss the importance of accurate calculations in experimental design and outcomes.

● Describe how the molar quantity of a substance is related to its molecular weight and calculate the molar quantity of various substances.

● Define the term hydrate and describe how hydrated compounds influence precipitation reactions.

● Predict and calculate the theoretical maximum amount of product produced in a precipitation reaction, using stoichiometry.

● Perform a precipitation reaction and measure the precipitate to calculate percent yield.

● Explain differences between theoretical and actual yield in a controlled experiment.

Time Allocation: 2.5 hours, plus an overnight drying period.

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Experiment Stoichiometry of a Precipitation Reaction

Materials Student Supplied Materials

Quantity Item Description 1 Bottle of distilled water 1 Dish soap 1 Roll of paper towels 1 Source of tap water

HOL Supplied Materials

Quantity Item Description 1 Digital scale, precision 1 Funnel, 70 mm 2 Glass beakers, 100 mL 1 Graduated cylinder, 25 mL 1 Pair of gloves 1 Pair of safety goggles 1 Experiment Bag: Stoichiometry of a Precipitation

Reaction:

1 - CaCl2•2H2O Calcium chloride, dihydrate - 2.5 g 1 - Filter paper, 12.5 cm 1 - Na2CO3 - Sodium carbonate - 2 g 1 - Weighing boat, plastic

Note: To fully and accurately complete all lab exercises, you will need access to:

1. A computer to upload digital camera images.

2. Basic photo editing software such as Microsoft Word or PowerPoint, to add labels, leader lines, or text to digital photos.

3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources.

Note: The packaging and/or materials in this LabPaq kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List form included in your LabPaq kit.

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Experiment Stoichiometry of a Precipitation Reaction

Background Chemical Equations

A chemical equation is an illustration of the reaction that occurs between two or more specific chemical compounds. Chemical equations use letters and numbers to represent the chemical elements and the amounts or ratios of those elements present in the compounds that are either participating in the reaction or a product of the reaction. For example, one methane molecule contains one carbon atom and four hydrogen atoms and is denoted as CH4. The chemical compounds that are present before a reaction occurs are called reactants, and the compounds produced from the reaction are called products. In addition to identifying the products and reactants in a balanced chemical reaction, a chemical equation will also quantitatively identify the proportion of reactants to products. This quantitative proportion is known as stoichiometry, and can be used to determine how much of each reactant is needed to produce a specific amount of each product. See Figure 1.

Figure 1. A balanced chemical equation. The chemical equation shows the chemical reaction between barium nitrate and copper sulfate. The equation shows that when 1 ion of barium

nitrate reacts with 1 ion of copper sulfate, 1 ion of barium sulfate and 1 ion of copper nitrate are produced.

As shown in Figure 1, chemical equations often denote the physical states of the reactants and products. The reaction in Figure 1 is a precipitation reaction, where two solutions are mixed and an insoluble substance (precipitate) forms, which is then able to be separated or removed from the solution. The (s) after BaSO4(s), denotes that a solid was formed as a product from the two aqueous (aq) reactants. The stoichiometry of a balanced chemical equation can be used to calculate the mass and number of moles of each reactant and each product in a chemical reaction.

Moles and the Periodic Table

A mole ( or mol) is a unit of measure, describing the amount of a chemical substance that contains as many atoms, ions, or molecules as there are in exactly 12 grams of pure Carbon (12C). One mole of a substance has 6.022 × 1023 atoms (for an element) or molecules (for a compound) or ions (for an ionic compound), and is equal to its molecular weight (formula mass). For example, the element nitrogen has a molecular weight of 14.01 grams, thus 1 mole of nitrogen is equal to 14.01 grams. Likewise, the compound H2O has a molecular weight of H + H + O (1.008 + 1.008 + 16.00), thus 1 mole of H2O is equal to 18.016 grams. The molecular weight of each element is found in the periodic table. See Figure 2.

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Experiment Stoichiometry of a Precipitation Reaction

Figure 2. Periodic Table of Elements. The molar mass of an element is equal to the mass in grams required to equal 1 mole of the substance.

Stoichiometric Quantities and Calculations

In addition to determining the amount of product formed in a reaction, stoichiometry can be used to determine how much of each reactant is required for all reactants to be used up at the same time. The quantities of reactants that are needed to fully react with one another at the same time are known as stoichiometric quantities. Stoichiometric quantities can be used to maximize the amount of product produced from the chemical reaction. For example, if you were performing the reaction in Figure 1 and had 3 grams of CuSO4, you can use the balanced chemical equation and stoichiometry to determine how many grams of Ba(NO3)2 you would need to create the maximum amount of BaSO4.

More specifically, to quantitatively calculate the maximum amount of product expected through a chemical reaction, you need only a balanced chemical equation, the atomic mass of each substance, and the quantity of substance available for only one of the reactants.

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Experiment Stoichiometry of a Precipitation Reaction

A step-by-step example of this process, using the balanced equation from Figure 1, is shown below:

Assuming there are only 5.7 grams of CuSO4 available, how many grams of Ba(NO3)2 are necessary to reach stoichiometric quantities? How many grams of solid BaSO4 are expected to be produced?

Step 1. Check to ensure that the equation is balanced. To do this, ensure that there is the same number of atoms from each element on both sides of the equation.

Step 2. Convert the 5.70 grams of CuSO4 to moles of CuSO4.

Step 3. Evaluate the molar ratio between CuSO4 and Ba(NO3)2.

The chemical equation states that for 1 mole of CuSO4, 1 mole of Ba(NO3)2 is needed for stoichiometric quantities. Using the information calculated in step 2, if there are 0.0357 moles of CuSO4, then 0.0357 moles of Ba(NO3)2 are required for a complete reaction.

Step 4. Convert moles of Ba(NO3)2 to grams of Ba(NO3)2.

This shows that 9.33 grams of Ba(NO3)2 are required to completely react with the 5.70 grams of CuSO4.

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Experiment Stoichiometry of a Precipitation Reaction

Step 5. Determine the amount (moles) of BaSO4 expected from the reaction.

The chemical equation states that for every 1 mole of CuSO4 used, 1 mole of BaSO4 is expected. This means that the 0.0357 moles of CuSO4 should produce 0.0357 moles of BaSO4.

Step 6. Convert moles of BaSO4 to grams of BaSO4.

To double-check the results of the calculations, the law of the conservation of mass can be applied. The Law of the Conservation of Mass states that the total mass, in a closed system, does not change as the result of reactions between its parts. Theoretically, this means that the total mass of the reactants should equal the total mass of the products. However, in practical experimentation, a system is seldom completely closed. As a result, one should realistically expect a slightly smaller amount of product, as the theoretical yield is rarely obtained. This deviation, from theoretical yield to actual yield, is called the percent yield and can be calculated.