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Đáp án:

`+) KMnO_4:` 1 nguyên tử K, 1 nguyên tử Mn, 4 nguyên tử O

`M=39+55+16.4=158` g/mol

`%K={39}/{158}.100=24,7%`

`%Mn={55}/{158}.100=34,8%`

`%O=100%-24,7%-34,8%=40,5%`

`+) Fe_2(SO_4)_3:` 2 nguyên tử Fe, 3 nguyên tử S, 12 nguyên tử O

`M=56.2+32.3+16.12=400` g/mol

`%Fe={56.2}/{400}.100=28%`

`%S={32.3}/{400}.100=24%`

`%O=100%-28%-24%=48%`

`+) NH_4NO_3:` 2 nguyên tử N, 4 nguyên tử H, 3 nguyên tử O

`M=2.14+4+3.16=80` g/mol

`%N={2.14}/{80}.100=35%`

`%H={4}/{80}.100=5%`

`%O=100%-35%-5%=60%`

`+) Ca_3(PO_4)_2:` 3 nguyên tử Ca, 2 nguyên tử P, 8 nguyên tử O

`M=40.3+31.2+16.8=310` g/mol

`%Ca={40.3}/{310}.100=38,7%`

`%P={2.31}/{310}.100=20%`

`%O=100%-20%-38,7%=41,3%`

`+) FeCl_3:` 1 nguyên tử Fe, 3 nguyên tử Cl

`M=56+35,5.3=162,5` g/mol

`%Fe={56}/{162,5}.100=34,5%`

`%Cl=100%-34,5%=65,5%`