The balanced equation is as below
2SO₂ +O₂ +2H₂O →  2 H₂SO₄
 The theoretical yield of H₂SO₄  is  294 grams
  calculation
step 1 :find the  moles of SO₂
 That is at STP  1  moles = 22.4 L
              ? moles = 67.2 L
by cross  multiplication
= (67.2 L Â x 1 Â moles) / 22.4 L Â = 3 Â moles
step 2: use  the  mole ratio to determine the  moles of H₂SO₄
from equation above the  mole ratio  of SO₂:H₂SO₄  is  2:2 = 1:1 therefore the moles of H₂SO₄  is also 3 moles
step 3: find the theoretical yield
theoretical yield= moles x  molar  mass
from periodic table the  molar mass of H₂SO₄  = (1 x2) + 32 + (16 x4) =98 g/mol
Theoretical yield = 3 moles x 98 g/mol = 294 grams
percent yield = Â actual yield/ theoretical yield x 100
=( 250g / 294 g) x 100 = 85.03%