chem calculation

For gases at the same temperature and pressure, volume ratio = mole ratio.

2H2(g) + O2(g) → 2H2O(g)
Volume ratio H2 : O2 = 2 : 1
Volume of H2 = 100 x 49% = 49 cm3
Volume of O2 needed = 49/2 = 24.5 cm3

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
Volume ratio CH4 : O2 = 1 : 2
Volume of CH4 = 100 x 28.5% = 28.5 cm3
Volume of O2 needed = 28.5 x 2 = 57 cm3

2COg) + O2(g) → 2CO2(g)
Volume ratio CO : O2 = 2 : 1
Volume of CO = 100 x 3% = 3 cm3
Volume of O2 needed = 3/2 = 1.5 cm3

Total volume of O2 needed
= 24.5 + 57 + 1.5
= 83 cm3

2007-09-07 01:57:09 補充：
看錯了！把題目的 100 dm^3 看成了 100 cm^3。更正：以上計算中，所有單位均為 dm^3，而不是 cm^3。而最後答案是『83 dm^3』。多謝 foflinanana 君指正錯誤。