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Slide 2 / 34 What Makes Hard Water Hard?

Have you ever had difficulty lathering soap or find that the scum in your shower constantly needs to be removed? These are signs of “hard water.” Soap doesn’t lather well in hard water because metal ions, such as Ca2+, form precipitates, creating “soap scum.” Because this happens, soap is a less effective cleanser in hard

• water. Hard water may contain various metal ions, including

Ca2+, Mg2+ and Fe2+.

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While the metal ions in water are generally harmless, hard water has other disadvantages, such as "boiler scale". Boiler scale is a scaly buildup of calcium

• carbonate. This scale can build up on the inside of water

pipes and can reduce the operating efficiencies of water

• heaters. Scale can also result in the failure of boiler

tubes as they become clogged. The best way to control the formation of boiler scale is through water pretreatment, such as installing a water

• softener. Water softeners typically replace the calcium

ions with soluble sodium ions.

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1 The addition of a water softener that replaces calcium ions with sodium ions has clear advantages in terms of extending the life of a water heater. What disadvantages could result when a household's water supply has an increase in sodium ions?

Students type their answers here

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In order to determine the hardness of water, each ion present in the water must be isolated separately. These ions or analytes can be isolated by precipitation reactions and analyzed through a process called gravimetric analysis. In completing the analysis the analyte is precipitated, purified, dried and massed. From the mass of the known composition of the precipitate, the amount of the analyte in the original solution can be calculated using stoichiometry.

What Makes Hard Water Hard? Slide 6 / 34 What Makes Hard Water Hard?

The goals of this lab are to: · Analyze water samples for their quantity of hardness through the principles of metal ion precipitation and separation. · Rank samples in order of increasing water hardness.

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2 A student wants to complete a gravimetric analysis of a water

sample to determine its quantity of hardness. Specifically, she wants to determine the mass percent of calcium in her sample. The calcium ion in this experiment is called the A substrate B solvent C analyte D specimen

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3 Which salt below could be added to her water sample to

precipitate out the calcium ion?

A Na2CO3 B NaC2H3O2 C NaNO3 D NaCl

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4 The student selects sodium carbonate to precipitate out her calcium ion. Write net ionic equation for the reaction

• f aqueous solutions of calcium chloride and sodium

carbonate.

Students type their answers here

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Once a determination has been made on which substance can be added to precipitate the analyte in question, the next issue to address is which laboratory techniques can be employed to ensure the analyte is free of impurities and sufficiently dried.

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5 The student is prepared to isolate the calcium ion from her 20 ml water sample using sodium carbonate. She

• btains a 50 ml sample of the 0.50 M sodium carbonate

and adds 3 mls then begins to collect her precipitate. Was this a good idea?

Yes No

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When completing a gravimetric analysis, an important consideration is that the analyte is completely

• precipitated. This can be accomplished by ensuring

that the analyte acts as the limiting reactant in the precipitation reaction.

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6 The student obtains another water sample and begins adding sodium carbonate. How will she know when she fully precipitated out all the calcium ion?

Students type their answers here

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Once the analyte is precipitated, it should be collected through a vacuum filtration system or through gravity filtration. All of the impurities should be removed from the precipitate through washing and the precipitate should be thoroughly dried.

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7 Once the student has collected her precipitate onto the filter paper, she washes her precipitate with deionized water, what is she attempting to remove?

A Ca2+ ions B Na+ and Cl- ions C Cl- ions only D Na+ ions only

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8 In the final step of her analysis, she thoroughly dries and masses her sample. The calcium carbonate precipitate weighs 1.251 g. What was the mass of calcium chloride in her original sample?

Students type their answers here

Slide 17 / 34 What Makes Hard Water Hard? Pre Lab Activity Slide 18 / 34

Materials:

Sodium carbonate, Na2CO3, Calcium chloride,CaCl2, weighing paper, balance, scoopula, 2 – 50 ml beaker, 25 ml graduated cylinder, deionized water, stirring bars, filter paper, vacuum filtration system (if available), funnel, permanent marker, watch glass, drying oven (if available), rubbing alcohol, goggles and aprons.

Setup:

· Each group of 3 students is given access to or given the materials. · If available your teacher will provide you with a vacuum filtration system and a drying oven to dry your precipitate. If these are not available then you can allow your sample to dry overnight.

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Pre Lab Activity - Procedure

1. Weigh 2 g of sodium carbonate in a clean, dry 50 ml beaker. 2. Weigh 2 g of calcium chloride in a clean, dry 50 ml beaker. 3. To each beaker, add about 20 ml of distilled water. Stir each with a glass stirring rod until each solid is dissolved. 4. Pour a small amount of the sodium carbonate solution into the beaker containing the calcium chloride solution. Stir and observe the mixture. Add some more of the sodium carbonate solution while stirring and observing. After all the sodium carbonate solution has been added, continue to stir this mixture for another couple of minutes. 5. Weigh out a piece of filter paper. 6. Fold the filter paper following your teacher's directions. 7. Set up the filtering apparatus. 8. Insert the filter paper into the filtering apparatus. Wet the filter paper with distilled water. 9. Pour the contents of your beaker slowly into the filter paper inside the funnel. Be careful as you pour so that none of the mixture flows out of the filter paper or the funnel. Use a wash bottle of distilled water to rinse the precipitate out of the beaker with small quantities

• f water. Use a little more water to wash the precipitate that is now collected in the filter

paper.

• 10. Using a permanent marker, label a clean, dry watch glass with your initials.

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• 11. Weigh the watch glass.
• 12. Carefully remove the filter paper with the precipitate from the filtering apparatus and set it
• n the watch glass. The filtrate, the solution remaining in the beaker, can be poured down the

drain.

• 13. If a drying oven is available allow the filter paper to dry for 10-15 minutes. If not, allow to

dry overnight and continue with step 16.

• 14. Carefully remove the warm watch glass. Use a metal scoop to break the precipitate into

small pieces. Set aside to cool.

• 15. Return the watch glass to the drying oven for another 5 minutes.
• 16. Carefully remove the watch glass out of the oven and set aside to cool.
• 17. Weigh the watch glass, filter paper and precipitate again.
• 18. Subtract the mass of the watch glass and filter from the mass of the watch glass, filter paper

and precipitate.

• 19. The precipitate and filter paper may be discarded in the waste basket. Wipe the permanent

marker off of the watch glass using some rubbing alcohol.

Pre Lab Activity - Procedure

Slide 21 / 34 Roles

Lab teams should consist of 3 students. The students should take turns and rotate through the roles listed below. 1. One person to follow the lab procedure and complete the lab set up and start the reaction. 2. One person (Collector) records the observations. 3. One person leads the completion of the calculations.

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Safety Warning:

· Always wear your goggles and aprons while completing this lab. · Also wear your goggles and aprons when you're near someone working with chemicals. · Familiarize yourself with the location of the eyewash and the emergency shower in the room. If you get any chemicals in your eye, use the eyewash. · If any of the chemicals come in contact with your skin rinse them off immediately with lots of water and let the instructor know what you spilled ---

• n your way to rinsing them off.

· Never mix chemicals that you haven't been told to mix. · Tell the instructor of any accidents immediately. · Keep food and drinks out of the laboratory work area.

Slide 23 / 34 Data Collection

1. Follow the procedure outlined on your lab sheet and collect mass data on sodium carbonate, calcium chloride, the filter paper, the watch glass, the filter paper and the watch glass and precipitate after the first heating and after the second heating. 2. Make observations concerning the formation of the precipitate.

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Gravimetric Guided Inquiry Lab Activity

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Materials:

Suggested list of materials: 50 ml samples of calcium carbonate solutions (unknown concentrations), 50 ml samples of 0.500M sodium carbonate, weighing paper, scoopula, 50 ml beakers, 25 ml graduated cylinder, deionized water, stirring bars, filter paper, vacuum filtration system (if available), funnel, permanent marker, watch glass, drying

• ven (if available), rubbing alcohol, goggles and aprons.

Setup:

· Each group of 2 students is given access to or given the materials. · If available your teacher will provide you with a vacuum filtration system to dry your precipitate. · If any additional lab equipment is needed students are to ask their instructors.

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· Design a procedure that will determine the unknown concentration of calcium ions as mg/l of CaCO3 in your water sample. · Write down a detailed step by step procedure and show your proposed procedure to your instructor before attempting the experiment. · Create your own data table from your instructor approved procedure. Make sure to include your water sample # in your data table and how many grams of precipitate were collected after each drying.

Gravimetric Inquiry Lab - Procedure

Slide 27 / 34 Roles

Lab teams should consist of 2 students. The students should take turns and rotate through the roles listed below. 1. One person to follow the lab procedure, complete the lab set up and start the reaction. 2. One person makes and notes observations. 3. One person leads the completion of the calculations.

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1. Determine how many grams of precipitate were collected. 2. Determine the hardness of your water sample in mg/l of CaCO3. 3. Based on the amount of CaCO3 produced, calculate the moles of calcium chloride in your original solution. 4. Outline the limiting reactant and justify your answer. 5. Determine the theoretical yield of CaCO3 based on your limiting reactant. 6. Calculate your % yield of CaCO3.

Calculations Slide 29 / 34

Analysis and Application Questions

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9 One can determine the limiting reactant in a reaction without a balanced chemical equation.

True False

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10 In order to successfully precipitate out the analyte in question it needs to be the limiting reactant in the reaction.

True False

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11 It is important to dry your sample twice to ensure all the water is removed because if water is present the mass of your precipitate will be less than it should be.

True False

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12 A student completes a gravimetric analysis of a water sample

containing calcium ions. After precipitating out the Ca2+ ions as calcium sulfate using sodium sulfate, they purified and dried their sample Their precipitate weighed 1.2230 g. How many grams of calcium chloride were present in their original sample? A 1.3559 g B 0.9970 g C 1.5441 g D 0.8865 g

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