Gravimetric Analysis Lab.pdf | Assignments Stoichiometry

Stoichiometry and Gravimetric Analysis

You are working for a company that makes water-softening agents for homes with hard water. Recently,

there was a mix-up on the factory floor, and sodium carbonate solution was mistakenly mixed in a vat with

an unknown quantity of distilled water. You must determine the amount of Na2CO3 in the vat in order to

properly predict the percentage yield of the water-softening product.

When chemists are faced with problems that require them to determine the quantity of a substance by mass,

they often use a technique called gravimetric analysis. In this technique, a small sample of the material

undergoes a reaction with an excess of another reactant. The chosen reaction is one that almost always

provides a yield near 100%. In other words, all of the reactant of unknown amount will be converted into

product. If the mass of the product is carefully measured, you can use stoichiometry calculations to

determine how much of the reactant of unknown amount was involved in the reaction. Then by comparing

the size of the analysis sample with the size of the original material, you can determine exactly how much of

the substance is present.

This procedure involves a double-displacement reaction between strontium chloride, SrCl2, and sodium

carbonate, Na2CO3. In general, this reaction can be used to determine the amount of any carbonate

compound in a solution.

Remember that accurate results depend on precise mass measurements. Keep all glassware very clean, and

do not lose any reactants or products during your lab work.

You will react an unknown amount of sodium carbonate with an excess of strontium chloride. After

purifying the product, you will determine the following:

 how much product is present.

 how much Na2CO3 must have been present to produce that amount of product.

 how much Na2CO3 is contained in the 575 L of solution

 the molarity of the Na2CO3 solution.

OBJECTIVES

 Observe the reaction between strontium chloride and sodium carbonate, and write a balanced

equation for the reaction.

 Demonstrate proficiency with gravimetric methods.

 Measure the mass of insoluble precipitate formed.

 Draw conclusions and relate the mass of precipitate formed to the mass of reactants before the

reaction.

 Calculate the mass of sodium carbonate in a solution of unknown concentration.

MATERIALS

 balance

 beaker, 250 mL

 distilled water

 drying oven

 filter paper

 funnel

 glass stirring rod

 graduated cylinder, 25 mL

 Na2CO3 solution

 Erlenmeyer flask, 250 mL

 SrCl2 solution

 water bottle

 dropper

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Stoichiometry and Gravimetric Analysis

You are working for a company that makes water-softening agents for homes with hard water. Recently, there was a mix-up on the factory floor, and sodium carbonate solution was mistakenly mixed in a vat with an unknown quantity of distilled water. You must determine the amount of Na 2 CO 3 in the vat in order to properly predict the percentage yield of the water-softening product.

When chemists are faced with problems that require them to determine the quantity of a substance by mass, they often use a technique called gravimetric analysis. In this technique, a small sample of the material undergoes a reaction with an excess of another reactant. The chosen reaction is one that almost always provides a yield near 100%. In other words, all of the reactant of unknown amount will be converted into product. If the mass of the product is carefully measured, you can use stoichiometry calculations to determine how much of the reactant of unknown amount was involved in the reaction. Then by comparing the size of the analysis sample with the size of the original material, you can determine exactly how much of the substance is present.

This procedure involves a double-displacement reaction between strontium chloride, SrCl 2 , and sodium carbonate, Na 2 CO 3. In general, this reaction can be used to determine the amount of any carbonate compound in a solution.

Remember that accurate results depend on precise mass measurements. Keep all glassware very clean, and do not lose any reactants or products during your lab work.

You will react an unknown amount of sodium carbonate with an excess of strontium chloride. After purifying the product, you will determine the following:

 how much product is present.  how much Na 2 CO 3 must have been present to produce that amount of product.  how much Na 2 CO 3 is contained in the 575 L of solution  the molarity of the Na 2 CO 3 solution.

OBJECTIVES

 Observe the reaction between strontium chloride and sodium carbonate, and write a balanced equation for the reaction.  Demonstrate proficiency with gravimetric methods.  Measure the mass of insoluble precipitate formed.  Draw conclusions and relate the mass of precipitate formed to the mass of reactants before the reaction.  Calculate the mass of sodium carbonate in a solution of unknown concentration.

MATERIALS

 balance  beaker, 250 mL  distilled water  drying oven  filter paper  funnel  glass stirring rod

 graduated cylinder, 25 mL  Na 2 CO 3 solution  Erlenmeyer flask, 250 mL  SrCl 2 solution  water bottle  dropper

Procedure

Put on safety goggles and lab apron.
Clean all of the necessary lab equipment with soap and water. Rinse each piece of equipment with distilled water.
Obtain about 100 mL of the unknown SrCl 2 solution in a plastic cup.
Obtain a piece of filter paper, measure the mass to the nearest 0.001 g, and record this value.
Measure about 15 mL of the Na 2 CO 3 solution into the graduated cylinder. Record this volume to the nearest 0.5 mL. Pour the Na 2 CO 3 solution into a clean, empty 250 mL beaker. Carefully wash the graduated cylinder, and rinse it with distilled water.
Measure about 25 mL of the SrCl 2 solution into the graduated cylinder. Pour the SrCl 2 solution into the beaker with the Na 2 CO 3 solution. Gently stir the solution and precipitate with a glass stirring rod.
Allow the precipitate to settle for 1 minute. Add a drop of SrCl 2 solution to determine if the reaction is complete. If not, add 10 additional mL of SrCl 2 solution. Repeat this step until you are certain the reaction is complete.
Obtain the 250 ml Erlenmeyer flask and funnel. Set up the filtration system and moist the filter paper.
Slowly pour the mixture into the funnel. Be careful not to overfill the funnel because some of the precipitate could be lost between the filter paper and the funnel. Once most of the liquid has been filtered, use a wash bottle to rinse the sides and bottoms of the beaker to transfer all of the precipitate to the filter.
Rinse the beaker several more times with small amounts of distilled water. Pour the rinse water into the funnel each time.
After all of the solution and rinses have drained through the funnel, slowly rinse the precipitate on the filter paper in the funnel with distilled water to remove any soluble impurities.
Carefully remove the filter paper from the funnel, and place it on a watch glass. Place the watch glass in the oven to dry.
Pour the filtrate in the other 250 mL beaker into the designated waste container. Clean up the lab and all equipment after use, and dispose of substances according to your teacher’s instructions. Wash your hands thoroughly after all lab work is finished and before you leave the lab.
After 24 hours: Measure and record the mass of the beaker with paper towel, filter paper, and precipitate to the nearest 0.001 g.
Dispose of the precipitate in a designated waste container. Clean up the lab and all equipment after use, and dispose of substances according to your teacher’s instructions. Wash your hands thoroughly after all lab work is finished and before you leave the lab.

How many grams of Na 2 CO 3 were present in the original 575 L sample? How many moles is this? What was the molarity of the original solution?
Why do you not need to know how much SrCl 2 was added in the reaction?
Would adding additional distilled water to the Na 2 CO 3 have affected the results in the experiment?
What affect would not rinsing your precipitate adequately enough have had on the results?
What affect would some of the precipitate passing through the filter paper during the filtration step have had on the results?

Gravimetric Analysis Lab.pdf, Assignments of Stoichiometry

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