Gr. 11. Titration problem (check if my answer is right)

[tuzzer]

1. The problem statement, all variables and given/known data
The acidic waste from a student's experiment needs to be disposed of, but pouring it down the sink would increase the impact of acid rain on the local water supply. Explain how you could determine the amount of limestone needed to neutralize the environmental impact of the experiment. (Assume the that limestone is mainly calcium carbonate.)

Write a balanced equations for any reactions you intend to use.


2. Relevant equations
H2SO4(aq) + CaCO3(s) --> H2O(l) + CO2(g) + CaSO4(s)



3. The attempt at a solution

Steps:

measure the concentration of the acid and the volume of the acid
grain the lime stone into powder and measure the weight
add methyl red or other indicator into the acid
slowly add the lime stone powder bit by bit until the indicator colour changes
measure the weight of the remaining lime stone and subtract it from the initial weight to obtain the mass of lime stone used to neutralize the acid
Determine the mass of lime stone needed to neutralize each Litre of the acid


Is that right?
The problem is that the salt produced from this chemical reaction is a solid. Would this affect the result?
And it might be hard to accurately put lime stone into the acid.
Is there a better way or a more correct way to do it?
Thanks

[GCT]

Here's a general suggestion,



Take a fixed quantity of the acid waste solution and add some limestone to it until the pH exceeds 7 (which can be detected by using an appropriate chemical indicator or litmus paper); you need to use a sufficient concentration of the limestone so that there would be enough base content to neutralize the particular amount of the waste acid solution. After the required amount of limestone has been found, take another sample with the same mass, and dissolve it in sufficient amount of water. Use this solution as a titrant for the waste acid solution of the same volume as before and determine the volume of it required to neutralize the acid (after this, calculate the corresponding mass of the limestone that was required to neutralize this acid solution by correlating the volume ratio with the original total mass of limestone that was dissolved to make the stock solution).

From all of this experimentation, find the value of grams of limestone required/L of the acid waste solution.

[tuzzer]

Does limestone dissolve in water? I thought it couldn't.
If it can dissolve in water, can I just take a certain mass of limestone and dissolve it in certain amount of water. Then determine the amount need to neutralize 1 L of the acid. then just multiply it by th concentration of the limestone solution to obtain the mass of limestone?

[ShawnD]

Does limestone dissolve in water? I thought it couldn't.
If it can dissolve in water, can I just take a certain mass of limestone and dissolve it in certain amount of water. Then determine the amount need to neutralize 1 L of the acid. then just multiply it by th concentration of the limestone solution to obtain the mass of limestone?

Calcium carbonate is chalk, so I think you are right when you say it won't dissolve.

The problem is asking about ratios, not exact values. You don't need to know any concentrations involved.
Take 50mL of acid waste and add a pH indicator that changes color somewhere around pH 7. While stirring, add small amounts of solid calcium carbonate until you reach an endpoint. Write down a ratio of calcium to acid, then multiply by your overall amount of acid. For example, 50mL of acid required 3g of calcium carbonate to neutralize, so 735mL of acid would require 44.1g of calcium carbonate (735mL x (3g/50mL) = 44.1g)

This is actually how industrial waste is neutralized. The sample amounts may be different but in the end you're still left pouring huge bags of weak acid or base into a vat full of liquid :biggrin: