Lab practical 2- water hardness

Question 1

What is water hardness a measure of?

Answer 1

the presence of magnesium and calcium in the water

Question 2

What is alkalinity?

Answer 2

waters capacity to resist acidic changes in pH

Question 3

What is total hardness?

Answer 3

Sum of Ca and Mg concentration expressed as calcium carbonate in mg L-1 or ppm.

Question 4

What is a complexometric titration?

Answer 4

Use of complexing agent (chelator like EDTA) to capture metal ions
Amount chelator used calculate amount of metal in water

Question 5

What is a chelate?

Answer 5

a compound containing a ligand bonded to a central metal atom at two or more points

Question 6

What is a ligand? (part of chelate)

Answer 6

ion or molecule which donates a pair of electrons to the central metal atom

Question 7

What is standardisation?

Answer 7

process of determining the exact concentration (molarity) of a solution

Question 8

What are the 3 steps of the water hardness practical?

Answer 8

1- Standardisation of EDTA Solution
2- Determination of Total Hardness (Mg2+ and Ca2+)
3- Determination of Calcium Hardness (Ca2+ only)

Question 9

What is the equation for moles?

Answer 9

moles= mass (g) / Mr

Question 10

What are the degrees of hardness and the CaCO3L-1 equivalent?

Answer 10

Soft = <50
Moderately soft = 50-100
Slightly hard = 100-150
Moderately hard = 150-200
Hard = 200-300
Very hard = >300

Question 11

What types of water were tested in the water hardness lab?

Answer 11

Distilled water
Tap water
Mineral water
“Sea” water (artificial)

Question 11

What is the hypothesis for the water hardness practical?

Answer 11

sea water will be hardest and distilled softest

Question 12

What is the aim of the water hardness practical?

Answer 12

use titration to determine the water hardness of 4 water samples

Question 13

Why are hard waters undesirable?

Answer 13

Require considerable amounts of soap to produce lather
Produce scale in industrial boilers, heaters, and hot-water pipes

Question 14

What are the ecological advantages of hard water?

Answer 14

neutralisation of acid deposition and the reduction of the solubility of toxic metals

Question 15

What weathered materials contribute to water hardness?

Answer 15

limestone
dolomite
gypsum

Question 16

How do you calculate Mg hardness? (water hardness practical)

Answer 16

Magnesium hardness = total hardness − calcium hardness

Question 17

What is a titration?

Answer 17

technique where a solution of known concentration is used to determine the concentration of an unknown solution

Question 18

What is the titrant and analyte? (titrations)

Answer 18

Titrant known solution
Analyte unknown solution

Question 19

What is the endpoint for titrations?

Answer 19

physical change (colour)

Question 20

What is the colour change expected from Eriochrome Black T indicator? (water hardness titration)

Answer 20

wine red to blue

Question 21

What materials were used in the water hardness practical?

Answer 21

• pH meter and combination or glass-calomel electrode
• Tap water, bottled mineral water, distilled water, artificial “sea
  water”.
• Primary standard CaCO3 solution, 0.004 M
• Standard disodium ethylenediaminetetraacetate dihydrate
• 0.1M NaOH
• Eriochrome Black T indicator solution, 0.5% (w/v) in ethanol, freshly
  prepared.
• Patton & Reeder’s indicator (mix with 100 times its weight of
  Na2SO4).
• Buffer solution. Dissolve 7.0 g NH4Cl in 57.0 mL of concentrated
  ammonia solution diluted to 1 L.
• Hydrochloric acid (HCl), concentrated

Question 22

What is step 1 of the standardisation of EDTA for the water hardness practical? (pipette and CaCO3)

Answer 22

Pipette, four 20 mL aliquots of the primary standard CaCO3 (0.004 M) solution into 250 mL conical flasks

Question 23

What is step 2 of the standardisation of EDTA for the water hardness practical? (titrate and EDTA Eriochrome black)

Answer 23

Titrate, with 0.004 M EDTA by adding 15 mL of the EDTA solution the burette, then 10 mL of the buffer solution followed by five drops
of the Eriochrome Black T

Question 24

What is step 3 of the standardisation of EDTA for the water hardness practical? (continuation of titration)

Answer 24

titration continue keep stirring until colour change
The first will act as a rough
then do 3 accurate titrations

Question 25

What is a rough titration?

Answer 25

the first titration done to give a rough idea of the endpoint to allow next titrations to be more accurate

Question 26

What is step 4 of the standardisation of EDTA for the water hardness practical? (conc EDTA and milligrams CaCO3)

Answer 26

avg of 3 accurate titrations
Use molar concentration formula
Then use moles equation substituting volume for mass

Question 27

What is the molar concentration equation?

Answer 27

(M1V1/n1)= (M2V2/n2)

Question 28

What does the formula for molar concentration mean? (M1V1 etc)

Answer 28

M1 and M2 are the moles of CaCO3 and EDTA (the unknown, that you want to calculate), V1 and V2 are the volumes of CaCO3 and EDTA used, respectively, and n1 and n2 are both 1 in this case, since 1 EDTA molecule reacts with 1 molecule of CaCO

Question 29

What is step 1 of the determination of total hardness? (water hardness practical- water sample, rough titration)

Answer 29

Pipette 20 mL water sample into 250mL conical flask, add 10 mL buffer and 5 drop of indicator
Do rough titration

Question 30

What is step 2 of the determination of total hardness? (repetition, titration)

Answer 30

Repeat titration with 20 mL water but add 3/4 expected titrant (based off rough end point)
Repeat several times for average

Question 31

What is step 3 of the determination of total hardness? (calculate)

Answer 31

Calculate the total hardness as mg L-1 CaCO3 equivalents

Question 32

What is the equation to calculate hardness as mg CaCO3L-1? (water hardness practical)

Answer 32

mg CaCO3 L-1 = 1000 * Vt * M/Vs

Question 33

What does the equation for hardness as mg CaCO3L-1 mean? (water hardness practical)

Answer 33

Vt= mL titrant needed for endpoint
M= mg CaCO3 equivalent to 1 mL EDTA
Vs= volume sample analysed

Question 34

What is step 1 of determining calcium hardness? (pH)

Answer 34

adjust pH of 20 mL water sample so between 12 to 12.25 using 1 M NaOH

Question 35

What is step 2 of determining calcium hardness? (Patton & Reeders)

Answer 35

Add 0.2g Patton & Reeder’s to solution and titrate
Repeat in several individual samples of same water for average

Question 36

What is step 3 of determining calcium hardness? (calculate)

Answer 36

Calculate Ca hardness expressed mg CaCO3L-1

Question 37

What is the formula for finding Ca hardness?

Answer 37

Ca (mg L-1) = 401 * Vt * M/Vs