Answer: A (SO₂). The percentage of sulfur by mass is the mass of sulfur in one formula unit divided by the \(M_r\), times 100%. Work each out. For SO₂, \(M_r = 32 + 2(16) = 64\), so %S \(= 32/64 = 50.0\%\). For H₂SO₄, \(M_r = 2 + 32 + 64 = 98\), so %S \(= 32/98 = 32.7\%\). For Na₂S, \(M_r = 46 + 32 = 78\), so %S \(= 32/78 = 41.0\%\). For PbS₂, \(M_r = 207 + 64 = 271\), so %S \(= 64/271 = 23.6\%\).
SO₂ has the highest percentage at 50.0%, so the answer is A. Tip: for a high percentage of an element, look for a formula where that element is heavy relative to the few other atoms present — in SO₂ only two oxygens compete with the sulfur.
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