Các quá trình trao đổi e:
$\mathop{Na}\limits^0\to \mathop{Na}\limits^{+1}+1e$
$\mathop{Ba}\limits^{0}\to \mathop{Ba}\limits^{+2}+2e$
$2\mathop{H}\limits^{+1}+2e\to \mathop{H_2}\limits^0$
$\mathop{O}\limits^0+2e\to \mathop{O}\limits^{-2}$
Bảo toàn e:
$n_{Na}+2n_{Ba}=2n_O+2n_{H_2}$
$\to a=n_{H_2}=\dfrac{n_{Na}+2n_{Ba}-2n_O}{2}$