How much sulphuric acid can be obtained from 5 kg of sulfur How much sulfuric acid $(\ce{H2SO4})$ can be obtained from 5 kg of sulfur $\ce{S8}$?
answer: 15.312 kg

To  make 1 molecules of $\ce{H2SO4}$ we need 1 atoms of S. 
Now coming to $\ce{S8}$. This gives us 8 atoms of S. so from this we can obtain 8 $\ce{H2SO4}$ molecules if we take 1 atom of $\ce{S8}$ and use it to make $\ce{H2SO4}$ molecules. So,

1 molecule of $\ce{S_8}$ $\ce{-&gt;}$ 8 molecules of $\ce{H2SO4}$
0.019 moles of $\ce{S_8}$   $\ce{-&gt;}$  $8 \times 0.019=0.15$ moles of $\ce{H2SO4}$ 

Formula mass of $\ce{H2SO4} = 1(2)+32+16(4) = 98$
we know the formula
for $$\text{Number of moles} = \frac{\text{Mass}}{\text{Formula Mass}}$$
So from this $\text{Mass}_\ce{H2SO4} = \text{Moles} \times \text{Formula Mass} = 0.15 \times 98=14.7$
Now you see that my answer doesn't match to the answer. What did I do wrong?
$\ce{S6}$, $\ce{S8}$, $\ce{S12}$ – does it make a difference or is it just a trick to make the question more complicated than it is?

What you know for sure is:


$M(\ce{S}) = 32.065\ \mathrm{g}\cdot \mathrm{mol}^{-1}$ 
$M(\ce{H2SO4}) = 98.079\ \mathrm{g}\cdot \mathrm{mol}^{-1}$


$\frac{5000\,\mathrm{g}}{32.065\,\mathrm{g}\cdot \mathrm{mol}^{-1}} = 155.93\,\mathrm{mol}$

$155.93\,\mathrm{mol} \cdot 98.097\,\mathrm{g}\cdot \mathrm{mol}^{-1} = 15.29\,\mathrm{kg}$, which is close enough to the answer.