Em tham khảo nha:
\(\begin{array}{l}
a)\\
CTHH:C{a_x}{O_y}\\
\text{ Theo quy tắc hóa trị } :II \times x = II \times y \Rightarrow x:y = 2:2 = 1:1\\
\Rightarrow CTHH:CaO\\
CTHH:A{l_a}C{l_b}\\
\text{ Theo quy tắc hóa trị }:III \times a = I \times b \Rightarrow a:b = 1:3\\
\Rightarrow CTHH:AlC{l_3}\\
b)\\
{M_{{H_2}O}} = 1 \times 2 + 16 = 18\,g/mol\\
{M_{A{l_2}{O_3}}} = 27 \times 2 + 16 \times 3 = 102\,g/mol\\
{M_{M{g_3}{{(P{O_4})}_2}}} = 24 \times 3 + (31 + 16 \times 4) \times 2 = 262\,g/mol\\
{M_{Ca{{(OH)}_2}}} = 40 + (16 + 1) \times 2 = 74\,g/mol\\
\end{array}\)
