- #1
Luckeris
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Homework Statement
Homework Equations
The Attempt at a Solution
I have no idea how to solve this. Any info would be helpful.
How to Determine the pH of a Water Sample?
- Thread starter Luckeris
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SUMMARY
This discussion focuses on determining the pH of a water sample using the mathematical definition of pH, which is pH = -log [H+]. The conversation highlights the importance of using molar concentration rather than mass concentration for accurate pH calculations. A specific example is provided where the pH is calculated as 4.638 based on a hydrogen ion concentration of 2.3*10-5 mol/L. Additionally, the discussion touches on related concepts such as alkalinity and water hardness, emphasizing their relevance in water quality analysis.
PREREQUISITES- Understanding of logarithmic functions and their applications in chemistry.
- Familiarity with molar concentration and its calculation.
- Basic knowledge of water chemistry, including definitions of pH, alkalinity, and hardness.
- Experience with complexometric titration techniques, particularly using EDTA.
- Learn about calculating pH using different ion concentrations in solutions.
- Research the principles of complexometric titration and its applications in measuring water hardness.
- Explore the definitions and implications of alkalinity in water quality assessment.
- Study the effects of calcium and magnesium salts on water properties and their measurement techniques.
Chemistry students, environmental scientists, water quality analysts, and anyone involved in water treatment and analysis will benefit from this discussion.
I have no idea how to solve this. Any info would be helpful.
- #2
Borek
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What is pH definition?
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- #3
Luckeris
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Quantitative measure of the strength of the acidity or alkalinity of a solution.
- #4
Borek
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That's s descriptive definition, there is also a specific, mathematical one, much more precise.
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- #5
Luckeris
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The mathematical definition of pH is a bit less intuitive but in general more useful. It says that the pH is equal to to the negative logarithmic value of the Hydrogen ion (H+) concentration, or
pH = -log [H+]
pH can alternatively be defined mathematically as the negative logarithmic value of the Hydroxonium ion (H3O+) concentration. Using the Bronsted-Lowry approach
pH = -log [H3O+]
pH values are calculated in powers of 10. The hydrogen ion concentration of a solution with pH 1.0 is 10 times larger than the hydrogen concentration in a solution with pH 2.0. The larger the hydrogen ion concentration, the smaller the pH.
when the pH is above 7 the solution is basic (alkaline)
when the pH is below 7 the solution is acidic
pH = -log [H+]
pH can alternatively be defined mathematically as the negative logarithmic value of the Hydroxonium ion (H3O+) concentration. Using the Bronsted-Lowry approach
pH = -log [H3O+]
pH values are calculated in powers of 10. The hydrogen ion concentration of a solution with pH 1.0 is 10 times larger than the hydrogen concentration in a solution with pH 2.0. The larger the hydrogen ion concentration, the smaller the pH.
when the pH is above 7 the solution is basic (alkaline)
when the pH is below 7 the solution is acidic
- #6
Borek
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Luckeris said:
OK, can you use information provided and this equation to calculate pH?
- #7
Luckeris
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pH = -log [H+]=-log[0.23*10-2]=2.638 ??
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- #8
Borek
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No, you need molar concentration, not mg/L.
- #9
Luckeris
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m=0.0023g
M(H)=1g/mol
n=m/M=0.0023mol
c=n/V=0.0023mol/l ?
M(H)=1g/mol
n=m/M=0.0023mol
c=n/V=0.0023mol/l ?
- #10
Borek
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Luckeris said:
This is not equivalent to 2.3*10-2mg, but other than that you are on the right track.
- #11
Luckeris
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Oh yeah mili- means *10-3 not *10-1
m=2.3*10-5g
M(H)=1g/mol
n=m/M=2.3*10-5mol
c=n/V=2.3*10-5mol/l
pH = -log [H+]=-log[2.3*10-5]=4.638
Is that it in terms of pH?
m=2.3*10-5g
M(H)=1g/mol
n=m/M=2.3*10-5mol
c=n/V=2.3*10-5mol/l
pH = -log [H+]=-log[2.3*10-5]=4.638
Is that it in terms of pH?
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- #12
Borek
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Yes. Perhaps too many significant figures, but that's correct approach.
Now, look for definitions of both alkalinity and hardness, and think how they apply.
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Now, look for definitions of both alkalinity and hardness, and think how they apply.
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- #13
Luckeris
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Alkalinity - http://www.ehow.com/how_5328969_calculate-alkalinity-caco.html
Hardness - Water hardness is a measure of the amount of calcium and magnesium salts dissolved in water. There are no health hazards associated with water hardness, however, hard water causes scale, as well as the reduced lathering of soaps. Complexometric titration is one of the best ways of measuring total water hardness. Ca2+ + EDTA4- → CaEDTA2-
Indicator (Eriochrome Black T) should be used to determine the end point of titration. Water hardness is usually reported in terms of mg/L of calcium carbonate which could by found from the mass of titrant used.
Hardness - Water hardness is a measure of the amount of calcium and magnesium salts dissolved in water. There are no health hazards associated with water hardness, however, hard water causes scale, as well as the reduced lathering of soaps. Complexometric titration is one of the best ways of measuring total water hardness. Ca2+ + EDTA4- → CaEDTA2-
Indicator (Eriochrome Black T) should be used to determine the end point of titration. Water hardness is usually reported in terms of mg/L of calcium carbonate which could by found from the mass of titrant used.
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- #14
Borek
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Luckeris said:
Good starting point, compare this with the data you are given.
- #15
Luckeris
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So I have Ca in mg/L which was found from CaCO3 + EDTA4- → CaEDTA2- + CO32- or not?
- #16
Borek
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Close, but not exact. You don't have solid CaCO3, but dissolved one. While calcium carbonate is weakly soluble, it is not completely insoluble. In fact, there are no completely insoluble salts.
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