# Variation of masses in lead-acid battery due to H2SO4 mass variation

1. Nov 9, 2009

### libelec

1. The problem statement, all variables and given/known data

In discharge mode, calculate the variation in mass of the compounds of a lead-acid battery if the sufuric acid mass decreases by 294g (3 moles).

2. Relevant equations

The semirreactions:

$$Pb \to P{b^{2 + }} + 2{e^ - }$$
$$4{H^ + } + Pb{O_2} + 2{e^ - } \to P{b^{2 + }} + 2{H_2}O$$

3. The attempt at a solution

The only thing I could deduce was that if the H2SO4 decreases in 3 moles, then this means that 3 moles of PbSO4 were formed, along with 3 moles of H2O (since the relation is 1 mole of each of the former formed for each mole of H2SO4 consumed).

But I can't figure out what would happen to the Pb or the PbO2.

I have the answers: Pb increases in 1,5 moles and PbO2 decreases in 1,5 moles.

Any ideas?

Thanks

2. Nov 10, 2009

### Staff: Mentor

Try with overall reaction equation.

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methods

3. Nov 10, 2009

### libelec

But the overall reaction:

$$Pb + Pb{O_2} + 2{H_2}S{O_4} \to 2PbS{O_4} + 2{H_2}O$$

doesn't tell me why the Pb mass increases and the PbO2 mass decreases.

4. Nov 10, 2009

### Staff: Mentor

Perhaps that means that the answer you were given is incorrect?

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methods

5. Nov 14, 2009

### libelec

Yes, such was the case. Thank you.

6. Nov 14, 2009

### Staff: Mentor

Not that I am surprised

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