[Generic Username]

How many grams of pyrite are required per litre of pH neutral water for the pyrite alone to lower the pH to 3? Pyrite oxidises to Iron(II)sulphate.

The answer is 60mg, though I haven't got much idea how it was arrived at. The reaction involved is:

2 FeS2(s) + 7 O2(g) + 2 H2O(l) → 2 FeSO4(aq) + 2 H2SO4(aq)

I have an inkling that the first step is working out the oxonium ion concentration from the pH:
[H3O+]=10^-pH ⇔ 
[H3O+]=10^-3  ⇔
[H3O+]=0.001M

Past that not much springs to mind. Perhaps divalence because of the sulphur, which would mean multiplying the concentration by two (I think). In trying to work out the amount of H2SO4 produced, I tried to use a formula I found elsewhere where the H2O is a limiting reactant:

Moles H2O (limiting reactant) = 1000 g / 18 g/mol= 55.56 moles H2SO4
Mass H2SO4 = 55.56 mol x 98 g/mol=5444 g

Which just seems an insane amount given that there was only a litre of water to begin with.

Any help greatly appreciated.

[thetada]

I think you're making the assumption that all of the water in the container will react, which seems unlikely. I would try to figure out the concentration of H2SO4 needed to reduce the pH to 3. The only problem is that's not a straightforward calculation. You could assume that the sulfuric acid completely dissociates (ie one mole of acid produces two moles of protons), even though that's not exactly what happens.

[AWK]

Mass H2SO4 = 55.56 mol x 98 g/mol=5444 g

Answer 60 mg is only approximately correct.

[Borek]

Assume sulfuric acid is completely dissociated - what concentration is required for the pH 3.0?

How much (moles) of sulfuric acid per 1 L of the solution?

How many moles of pyrite required to produce this amount if sulfuric acid? (Trivial stoichiometry, based on the reaction equation you have posted).

Convert moles to mg, done.

[Generic Username]

Assume sulfuric acid is completely dissociated - what concentration is required for the pH 3.0?

How much (moles) of sulfuric acid per 1 L of the solution?

How many moles of pyrite required to produce this amount if sulfuric acid? (Trivial stoichiometry, based on the reaction equation you have posted).

Convert moles to mg, done.

2 FeS2(s) + 7 O2(g) + 2 H2O(l) → 2 FeSO4(aq) + 2 H2SO4(aq)

7 - 2 = 5
m = M * n, where M = 119.975 g/mol
m = 119.975 * 5
m = 0.5998 g
No, I was too quick off the mark in assuming I'd finally done it.  0.60g is not 60mg. 

[AWK]

You calculated [H3O+]=0.001M correctly. Its means you have  0.001/2 M H2SO4 assuming its complete dissociation (Borek hint).
Looking at reaction you have the same amount of FeS₂ moles which is 60 mg (

trivial stoichiometry

can be even done without calculator).
But in fact you have Fe(HSO₄)₂. HSO₄^-  at this concentration (10^-3 M) dissociates almost completely, I guess ~95 %.
Hence you have in solution H₃O⁺ = HSO₄^- and SO₄^2- and pH calculation without approximation is more complicated (almost complete dissociation of HSO₄^- is practically equivalent to your reaction) though this approximation gives a few percent error (a few mg more).

7 - 2 = 5
m = M * n, where M = 119.975 g/mol
m = 119.975 * 5
m = 0.5998 g

[Borek]

2 FeS2(s) + 7 O2(g) + 2 H2O(l) → 2 FeSO4(aq) + 2 H2SO4(aq)

7 - 2 = 5

While 7-2=5 is algebraically correct, it is in no way related to the problem.

Coefficients of the reaction equation tell you what is the molar RATIO between substances. How many moles of sulfuric acid will be produced when two moles of the pyrite react?

Compare http://www.chembuddy.com/?left=balancing-stoichiometry&right=stoichiometric-calculations

[Generic Username]

2 FeS2(s) + 7 O2(g) + 2 H2O(l) → 2 FeSO4(aq) + 2 H2SO4(aq)

7 - 2 = 5

While 7-2=5 is algebraically correct, it is in no way related to the problem.

Coefficients of the reaction equation tell you what is the molar RATIO between substances. How many moles of sulfuric acid will be produced when two moles of the pyrite react?

Compare http://www.chembuddy.com/?left=balancing-stoichiometry&right=stoichiometric-calculations

True. I can now see that it's clearly wrong and completely unrelated to anything I've read about stoichiometry so far. I need to step away from this and everything chemistry related for a while and come back to it later. Thanks for the help, guys.