# Amount of 6M H2SO4 to lower pH of pool from 7.0 to 4.0

• Terp
In summary, 6 M H2SO4 must be added to lower the water pH to 4.0 so that the plaster liner in the pool can be cured.
Terp
[SOLVED] Amount of 6M H2SO4 to lower pH of pool from 7.0 to 4.0

## Homework Statement

A new swimming pool is filled with fresh, clean water with a pH of 7.0. The width is 25 ft, the length is 60 ft, and the depth linearly ranges from 3.5 to 9 ft. How much 6 M H2SO4 must be added to reduce the water pH to 4.0 so that the plaster liner in the pool can be cured?

## Homework Equations

kw = ka*kb, pKa = -log(Ka) and for pOH and pH, any the usual concentration formulas.

## The Attempt at a Solution

I'm honestly not entirely sure how to tackle this. I first started off by finding things I may need. I found the capacity of the pool to be 265,643 liters. The molecular weight of H2SO4 is 98.065 g/mol. Ka of H2SO4 is 10^3.

If you guys could bump me in the right direction it'd be appreciated. Thanks!

Last edited:
Nevermind, got it figured out :).

Well since you figured it out, maybe you could tell me. I have not got to that section of the book yet in school, so I just read ahead. The way I saw it being done was this: Since you want a pH of 4, I computed Molar by using wanted pH 10^-4 = .0001 Molar. Then I took the equation M1V1=M2V2 solved for V1 so I got (.0001M * 265643L)/6M =4.43L
I have a feeling that is not correct because H2SO4 is diprotic. What answer did you come up with?

Well, if you want a pH of 4 then you will need a [H+] = 1 x 10^-4 M ([H+] = antilog (-pH)

I assumed that the moles of H+ ion that will be present in the swimming pool after adding the acid will be the same as the number of moles of H+ provided by the acid. So I came up with an equation:

moles of H+ from acid = moles of H+ in swimming pool after the acid is added

H2SO4 is for all practical purposes completely ionized into the H+ ions and SO4-2 ions according to the equation:
H2SO4 --> 2 H+ + SO4-2

so the moles of H+ for acid = volume of acid added x molarity of the acid x 2 H+/acid molecule
= x L (6mol/L)(2 H+/molecule)

the moles of H+ in the pool = molarity of H+ in the pool x volume of the pool
= (1 x 10^-4 M)( 2.66 x 105) L

Plugging the values in for H+ yields:
x L(6mol/L)(2 H+/molecule) = (1x 10^-4M)(265643L)

I just solved for X and got 2.2L. I'm not 100% this is correct but it makes sense to me. It's basically a molarity balance thing. Moles of [H+] for the ace = moles of [H+] after it's in the water. I think what you did is correct for one stage of the dissociation, but not both, since the only difference is that I multiplied the 6M by two from the chemical equation above.

Good luck and godspeed. Engineering majors like me need it! haha

Last edited:
Yeah that makes sense. I think I messed up by not accounting for the H2, hence my number was 2x. Thanks for clarification. Funny you ask for help and end up giving it :)

## Q1: What is the formula for calculating the amount of 6M H2SO4 needed to lower the pH of a pool from 7.0 to 4.0?

The formula for calculating the amount of 6M H2SO4 needed is:
Amount of 6M H2SO4 = (Volume of pool x (desired pH - current pH)) / 6

## Q2: How do I determine the volume of my pool for the calculation?

The volume of the pool can be determined by multiplying the length, width, and average depth of the pool. This will give you the total volume in cubic feet. Then, divide by 7.48 to convert to gallons.

## Q3: Can I use a different concentration of H2SO4 to achieve the same results?

Yes, you can use a different concentration of H2SO4, but the amount needed will vary. You can use the formula:
Amount of acid needed = (Volume of pool x (desired pH - current pH)) / concentration of acid

## Q4: Is it safe to add 6M H2SO4 directly to my pool?

No, it is not safe to add concentrated acid directly to the pool. It should be diluted first in a separate container and then added to the pool slowly while the pool pump is running to ensure proper mixing and distribution.

## Q5: How long should I wait before retesting the pH after adding 6M H2SO4 to my pool?

You should wait at least 24 hours before retesting the pH after adding 6M H2SO4 to your pool. This will give the acid enough time to fully mix and react with the pool water.

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