Mỗi phát biểu nào sau đây là đúng hay sai?

a. Số oxi hoá của nguyên tử trong bất kì một đơn chất hoá học nào đều bằng 0

Chọn đáp án B

$n_{O_2}\text{ = 0,125 mol; }\stackrel{BTKL}{\to }{\text{ m}}_X\text{ = 9,1 }-\text{ 0,125.32 = 5,1 gam}$

$\left.\begin{array}{l}Mg\text{ (a) }\stackrel{}{\to }{\text{ Mg}}^{+2}\text{ + 2e (2a) }\\ Al\text{ (b) }\stackrel{}{\to }{\text{ Al}}^{+3}\text{ + 3e (3b)}\end{array}\right|\left|\begin{array}{l}{O}_2\text{ + 2.2e }\stackrel{}{\to }2O^{-2}\\ \text{ 0,125 }\to \text{ 0,5}\end{array}\right.$

$\Rightarrow \left\{\begin{array}{l}\stackrel{BT\text{ e}}{\to }\text{ 2a + 3b = 0,5}\\ \stackrel{m_X}{\to }\text{ 24a + 27b = 5,1}\end{array}\right.\Rightarrow \left\{\begin{array}{l}\text{a = 0,1}\\ \text{b = 0,1}\end{array}\right.\stackrel{}{\to }{\text{ m}}_{Mg}\text{ = 2,4 gam}$

$n_X\text{ = 0,1 mol; }\stackrel{BTKL}{\to }{\text{ m}}_X\text{ = 8,84 }-\text{ 2,52 = 6,32 gam}$

$\left.\begin{array}{l}Mg\stackrel{}{\to }{\text{ Mg}}^{+2}\text{ + 2e }\\ Al\stackrel{}{\to }{\text{ Al}}^{+3}\text{ + 3e }\end{array}\right|\left|\begin{array}{l}C{l}_2\text{ (x) + 2.1e (2x) }\stackrel{}{\to }2C{l}^{-}\\ O_2\text{ (y) + 2.2e (4y) }\stackrel{}{\to }2O^{-2}\end{array}\right.$

$\Rightarrow \left\{\begin{array}{l}\stackrel{n_X}{\to }\text{ x + y = 0,1}\\ \stackrel{m_X}{\to }\text{ 71x + 32y = 6,32}\end{array}\right.\Rightarrow \left\{\begin{array}{l}\text{x = 0,08}\\ \text{y = 0,02}\end{array}\right.\stackrel{}{\to }\left\{\begin{array}{l}\%V_{C{l}_2}\text{ = 80\%}\\ \%V_{O_2}\text{ = 20\%}\end{array}\right.$

$\stackrel{}{\to }{\text{ n}}_e\text{ = 2x + 4y = 0,24 mol}$