a.

`\text{K (Kali)}` hoá trị `\text{I}`, `\text{O (Oxi)}` hoá trị `\text{II}`

`->` Giao hoán hoá trị: `\text{K₂O}`

`\text{M(K) = 39}` `->` `\text{2K = 2 × 39 = 78}

`\text{M(O) = 16}`

`\text{M(K₂O) = 78 + 16 = 94}`

$$ \%K = \dfrac{78}{94} \times 100 \approx 82.98\% $$

$$ \%O = \dfrac{16}{94} \times 100 \approx 17.02\% $$

CTHH: `\text{K₂O}`

`\text{%K ≈ 82.98%, %O ≈ 17.02%}`

b.

`\text{Al}` (nhôm) hoá trị `\text{III}`, nhóm `\text{CO₃}` (cacbonat) hoá trị `\text{II}`

`->` Giao hoán hoá trị: `\text{Al₂(CO₃)₃}`

`\text{M(Al) = 27}` `->` `\text{2Al = 54}`

`\text{M(C) = 12 × 3 = 36}`

`\text{M(O) = 16 × 9 = 144}`

`->` `\text{M(Al₂(CO₃)₃) = 54 + 36 + 144 = 234}`

$$ \%Al = \dfrac{54}{234} \times 100 \approx 23.08\% $$

$$ \%C = \dfrac{36}{234} \times 100 \approx 15.38\% $$

$$ \%O = \dfrac{144}{234} \times 100 \approx 61.54\% $$

CTHH: `\text{Al₂(CO₃)₃}`

`\text{%Al ≈ 23.08%, %C ≈ 15.38%, %O ≈ 61.54%}`

c.

`\text{Ca}` (canxi) hoá trị `\text{II}`, nhóm `\text{H₂PO₄}` hoá trị `\text{I}`

`->` Giao hoán hoá trị: `\text{Ca(H₂PO₄)₂}`

`\text{M(Ca) = 40}`

`\text{M(H₂PO₄) = 1×2 + 31 + 16×4 = 2 + 31 + 64 = 97}`

`->` `\text{2 × 97 = 194}`

`->` `\text{M(Ca(H₂PO₄)₂) = 40 + 194 = 234 % 𝐶 𝑎 = 40 234 × 100 ≈ 17.09 %}`

$$ \%Ca = \dfrac{40}{234} \times 100 \approx 17.09\% $$

$$ \%H = \dfrac{2 \times 2}{234} \times 100 = \dfrac{4}{234} \times 100 \approx 1.71\% $$

$$ \%P = \dfrac{2 \times 31}{234} \times 100 = \dfrac{62}{234} \times 100 \approx 26.50\% $$

$$ \%O = \dfrac{2 \times 4 \times 16}{234} = \dfrac{128}{234} \times 100 \approx 54.70\% $$

CTHH: `\text{Ca(H₂PO₄)₂}`

`\text{ %Ca ≈ 17.09%, %H ≈ 1.71%, %P ≈ 26.50%, %O ≈ 54.70%}`