How to Determine the pH of a Water Sample?

[Luckeris]

Homework Statement

Homework Equations

The Attempt at a Solution

I have no idea how to solve this. Any info would be helpful.

 

[Borek]

What is pH definition?

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[Luckeris]

Quantitative measure of the strength of the acidity or alkalinity of a solution.

 

[Borek]

That's s descriptive definition, there is also a specific, mathematical one, much more precise.

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[Luckeris]

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

 

[Borek]

pH = -log [H+]

OK, can you use information provided and this equation to calculate pH?

 

[Luckeris]

pH = -log [H⁺]=-log[0.23*10^-2]=2.638 ??

 

[Borek]

No, you need molar concentration, not mg/L.

 

[Luckeris]

m=0.0023g
M(H)=1g/mol
n=m/M=0.0023mol
c=n/V=0.0023mol/l ?

 

[Borek]

m=0.0023g

This is not equivalent to 2.3*10^-2mg, but other than that you are on the right track.

 

[Luckeris]

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?

 

[Borek]

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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[Luckeris]

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⁺ + EDTA^4- → CaEDTA^2-
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.

 

[Borek]

Water hardness is a measure of the amount of calcium and magnesium salts dissolved in water.

Good starting point, compare this with the data you are given.

 

[Luckeris]

So I have Ca in mg/L which was found from CaCO₃ + EDTA^4- → CaEDTA^2- + CO₃^2- or not?

 

[Borek]

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.