General Chemistry Prep w/Lab
Lab Exercise: Mass Relationships in a Chemical Reaction
In this lab exercise you will learn to prepare and collect a precipitate formed in the following double replacement reaction:
Calcium chloride (aq) + Sodium carbonate (aq) > Calcium carbonate (s) + Sodium chloride (aq)
You will also practice calculating the percent yield in a reaction.
A balanced chemical equation expresses the relationship between the reactants and products of a chemical reaction in a quantitative manner, as well as qualitative. By means of a correctly written equation we can determine the yield of a product that would be expected from a known quantity of reactant. For example, when writing the chemical reaction
2 H2 + O2 > 2 H2O
In this experiment you will add excess sodium carbonate to a solution containing a measured mass of calcium chloride, precipitating calcium carbonate. Since the excess sodium carbonate remains in the solution, and thus on the precipitate, you will rinse it away, capturing any precipitate of calcium carbonate which might escape by filtering the rinse water. After allowing the product to dry you will determine its mass and calculate the percent yield.
In this experiment you will be working with a lab partner.
Clean and dry a 150 mL beaker. Put your initials on the ground glass circle with a pencil to identify the beaker later. Determine the mass of the beaker and a short stir rod to the nearest 0.001 grams, and then add approximately 1.5 grams of calcium chloride. Again determine the mass to the nearest 0.001 gram. Add approximately 15 mL of deionized water to the beaker, and stir until the salt is dissolved. (Make sure you feel the bottom of the beaker — calcium chloride has a high heat of solution!)
Write and balance the equation for the double replacement reaction of calcium chloride with sodium carbonate. Calculate the amount of sodium carbonate needed to react with your amount of calcium chloride used. (Check your calculations with your instructor before proceeding.) Measure out, in a second container, an amount of sodium carbonate in slight excess of the amount calculated for complete reaction. Add about 15 mL of deionized water to the sodium carbonate and stir until dissolved, which will take a while.
Add the sodium carbonate solution to the calcium chloride solution, making sure that the beaker you weighed is the one in which the reaction takes place. Stir the resulting solution for a short while, and then gently warm the mixture for about ten minutes using a hot plate set to 200°. The heating allows the particle size to grow, which facilitates decanting.
While the solution is heating select a piece of filter paper and determine its mass to the nearest 0.001 gram. Fold the filter paper as illustrated by your instructor, moisten it with a little deionized water, and place it in a funnel. Your instructor will also demonstrate the proper setup for filtering.
Allow the mixture to cool to room temperature, and then decant the liquid from the beaker containing the precipitate of calcium carbonate into the funnel, being careful to retain as much of the product in the original beaker as possible. Use your stir rod to aid in the transfer as illustrated by your instructor. It is very important to avoid filling the filter cone more than 2/3 full in order to avoid loss of the precipitate.
Wash the precipitate in the beaker with about 10 mL of deionized water. Allow the solid to settle, and decant the liquid. Repeat this wash twice more with additional 10 mL portions of deionized water, decanting each time. Finally, wash the filter paper and any solid there with 5 mL of deionized water, and then allow it to drain completely.
Place the filter paper in the beaker with the precipitate and stir rod, and then place everything in the oven (set to 45°C) to dry until the next lab period. Upon your return, weigh the beaker containing the precipitate, filter paper and stir rod, and determine the mass of calcium carbonate formed.
Analysis and Calculations
% Yield = Experimental Yield
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