Question

What mass of oxygen is needed for the complete combustion of 7.70×10−3gg of methane

Answer 1

\[CH_4+2O_2~~\implies~~CO_2+2H_2O\]
which means for every mole of methane, you need 2 moles of oxygen for complete combustion.

Molar masses:
\[\begin{array}{c|c}
CH_4& 12+4(1)=16\frac{\text{g}}{\text{mol}}\\
\hline
O_2& 2(16)=32\frac{\text{g}}{\text{mol}}
\end{array}\]
\[(7.70\times10^{-3}\text{g})\times\frac{1\text{ mol}}{16\text{ g}}=x\text{ moles of }CH_4\]
which means you'll need \(2x\) moles of \(O_2\). Convert to mass.