`+` `M_{NaOH} = 23 + 16 + 1 = 40` (amu)

`=> % O = {16}/{40} = 40%`

`+` `M_{Na_2SO_4} = 23 . 2 + 32 + 16 . 4  = 142` (amu)

`=> % O = {64}/{142} ~~ 45,1%`

`+` `M_{CaCO_3} = 40 + 12 + 16 . 3 = 100` (amu)

`=> % O = {48}/{100} = 48%`

`+` `M_{H_3PO_4} = 3 . 1 + 31 + 4 . 16 = 98` (amu)

`=> % O = {64}/{98} ~~ 65,31%`

`=>` Chọn $CaCO_3$

`\color{#1AD5F7}{⋆⟡}\color{#1AD5F7}{C}\color{#4DA6E6}{h}\color{#668EDD}{i}\color{#8077D5}{p}\color{#995FCD}{p}\color{#EA2F90}{⟡⋆}`