Lab 1

What Makes Hard Water Hard?

Unit 1⏱ 90 minGuided Inquiry

Overview

How can the hardness of water be determined from a given sample? What techniques and calculations are required to determine water’s hardness?

Objectives

How can the hardness of water be determined from a given sample? What techniques and calculations are required to determine water’s hardness?
Connect observations to AP Chemistry models and calculations.

⚠️ Wear protective goggles and an apron. Follow your teacher’s safety directions for all chemicals and equipment.

Materials Needed

-mL samples of calcium chloride solutions (unknown concentrations) -mL samples of 0.50 M sodium carbonate solutions -mL beaker glass stir rod labeling tape -mL graduated cylinder pipettes ring stand utility clamp Buchner funnel filter paper Erlenmeyer vacuum flask with hose balance distilled water bottle Ice water Wear protective goggles and an apron throughout the laboratory activity Be aware of the proper techniques in using vacuum filtration

Procedure

Design a procedure that will determine the unknown concentration of Ca2+ ions as mg of CaCO3 per Liter of solution in a 50 mL sample of water.

Write down a detailed step by step procedure.

Show your proposed procedure to your teacher before attempting the experiment yourself.

You may use all of the equipment and chemicals listed above.

Pre-Lab Questions

1. Answer the objective of the lab and explain HOW you calculated your results. What was the hardness of your original water sample (before concentrating it)? Based on your results, quantitatively determine if your water sample soft, moderate or hard by comparing to water softening values. What specifically makes water “hard”? What detrimental effects can occur from having hard water in your home?

Post-Lab Questions

1. Why was the precipitate heated twice in the drying oven? If you forgot to wash the precipitate that was first collected with distilled water before drying it, would the mass of the precipitate be larger or smaller? Explain. From what you read in the introduction, if the water sample you were given came from your home, would a water softener be recommended? Why or why not? Based on the results of this experiment, if 50.0 mL of 0.60 M Na2CO3 was used instead of 50.0 mL of 0.50 M Na2CO3 would the amount of precipitate produced be more, less or the same as what was produced in this lab. Explain. If excess Na2SO4 (aq) is added to a 42.53 mL sample of Ba(NO3)2 (aq), what is the formula for the precipitate? In question #5, if the precipitate that formed was 3.046 grams, what was the molarity of the Ba(NO3) 2 ? A 5.000 gram mixture contains strontium nitrate and potassium bromide. Excess Pb(NO3) 2 (aq) is added to precipitate out 0.7822 grams of PbBr2 (s). a) What is the % by mass of the potassium bromide in the original mixture? b) What is the percent by mass of the strontium nitrate in the original mixture?