A metal M of equivalent mass E forms an oxide of molecular formula M_xO_y. The atomic mass of the metal is given by the correct equation

M_x​O_y​

Valence of oxygen is −2

Valance of y of oxygen is −2y

Let the valency of M is +P

Charge on x molecule of M is +Px

Compound is neutral

$\therefore$ −2y + Px = 0

⇒P = $\frac{2\mathrm{y}}{\mathrm{x}}$​electrons

Valence of metal M is $\frac{2\mathrm{y}}{\mathrm{x}}$, atomic mass of M is $\frac{2\mathrm{y}}{\mathrm{x}}\mathrm{E}$

Hence, atomic mass of M is $\frac{2\mathrm{yE}}{\mathrm{x}}$​.