Gravimetric Analysis of Water Hardness Lab
In this lab you will determine how hard our tap water is by analyzing the calcium chloride content and comparing
that content to a water hardness scale. To accomplish this, gravimetric technique will be performed for a standard
solution containing calcium chloride and the results will be used to analyze the tap water, containing an unknown
amount of calcium chloride.
Goggles and apron should be worn at all times in lab.
Suggested Materials:
2 Filters
Funnel
Ring Stand
Ring Clamp or Funnel Support
1.0 M Calcium Chloride
1.0 M Sodium Carbonate
100 mL Beaker
250 mL Beaker
600 mL or Larger Beaker
25 mL Graduated Cylinder
100 mL Graduated Cylinder
Wash Bottle with Deionized Water
Stir Rod
Balance
Introduction to guided inquire lab.
For this lab you will not be given step by step procedures to follow. It is your task to work with your partner to
determine your own procedure for obtaining the necessary data that will enable you to determine the hardness of tap
water.
To get you started, a list of suggest materials is above, and a chart showing the necessary data that needs to be
collected or calculate is below.
Requirements of Your Procedures:
1. Must address the reaction of a standard (known) solution of calcium chloride mixed with sodium carbonate in
such a way as to obtain the mass of the precipitate that is caught on the filter. Only 20.0 mL of each solution
should be used.
2. Must address the reaction of tap water mixed with sodium carbonate in such a way as to obtain the mass of the
precipitate that is caught on the filter. 200.0 mL of tap water should be used with 20.0 mL of sodium carbonate.
3. Must address cleaning up the lab equipment as you proceed and the final disposal of the chemicals in the proper
manner.
4. You may write your procedures in pencil, and must get it checked by Mr. Belland before you may begin.
Mass of “standard” filter paper
Mass of “standard” filter paper and solid
Calculated mass of “standard” solid (actual)
Mass of “tap” filter paper
Mass of “tap” filter paper and solid
Calculated mass of “tap” solid (actual)
Lab Report Requirement:
In a neat, well written, grammatically correct report that is written in ink or typed include the following:
1. The purpose of this lab, in detail
2. The procedures you followed, step by step
3. The data table and the data to complete it.
4. Detailed completion of the Calculations and Analysis and Error Analysis sections
Calculations and Analysis:
1. For the “standard”, 20 mL of 1.0 M calcium chloride and sodium carbonate were used. Calculate the
number of moles of each of these. Show your work, neatly, and label everything with the correct units.
2. Write a balanced equation for the reaction between calcium chloride and sodium carbonate.
3. How many grams of CaCO3 should have formed? Show your work, neatly, and label everything with the
correct units.
4. Calculate the percent yield for the “standard” reaction. Show the formula, then rewrite it with the numbers
and units in the correct places, and then calculate the answer.
5. Using the percent yield from calculation #5, and the actual “tap” mass from the data table, calculate the
theoretical amount of mass of CaCO3 that should have formed with the tap water. To do this the percent
yield formula gets rearranged to solve for theoretical amount. Show the formula, then rewrite it with the
numbers and units in the correct places, and then calculate the answer.
6. Assume the source of calcium in the tap water is calcium chloride, which you then reacted with sodium
carbonate. Write a balanced equation for this reaction.
7. How many moles of calcium chloride were in the tap water?
8. What is the concentration, in Molarity, of the calcium chloride in the tap water?
9. Your calculation from calculation #9 expresses the hardness of the tap water, but in molarity. Most water
hardness scales are expressed in parts per million, ppm. To convert the molarity of calcium ion into parts
per million, the ratio is 1 M = 40,000 ppm. What is the hardness of the tap water in ppm?
10. Below is a water hardness scale. What is the classification of our tap water?
PPM
Classification
< 17.1
Soft
17.1-60
Slightly Hard
60-120
Moderately Hard
120-180
Hard
>180
Very Hard
Error Analysis:
1. The volumes and concentrations of the standard solutions were in a 1:1 ratio, but the volumes of the tap
water to standard solution was in a 200:20 ratio. Green chemistry’s goal is to use only as little chemicals as
needed. Explain why only 20 mL of the sodium carbonated was needed, even with 200 mL of tap water.
2. What proof in your lab supports that the tap water contained the limiting reactant and it was not the sodium
carbonate? Explain your reasoning.
A student was not instructed how much standard sodium carbonate to react with the tap water and chooses an
amount that makes the sodium carbonate the limiting reactant. Would the calculated results of the water’s hardness
increase (more hard), decrease (less hard), or remain unchanged. Explain your reasoning.