72. C
`Ca + 2H_2O -> Ca(OH)_2 + H_2`
`x.........................x.............x`
`CaO + H_2O -> Ca(OH)_2`
`y.........................y`
`CaC_2 + 2H_2O -> Ca(OH)_2 + C_2H_2`
`z...........................z..................z`
`overline(Z) = (2x+26z)/(x+z)=7.2=14` (1)
Đốt Z:
` C_2H_2 + (5)/(2)O_2 -> 2CO_2 + H_2O`
`z.................................2z`
` H_2 + (1)/(2)O_2 -> H_2O`
Cho s/p cháy qua Y:
`CO_2` hết; `Ca(OH)_2` dư
`Ca(OH)_2 =x+y+z` (mol)
`Ca(OH)_2 + CO_2 -> CaCO_3 + H_2O`
`2z................2z..........2z`
`=> m↓ = 100.2z=40 => z=0,2` (mol)
Thay `z=0,2` vào (1) `=> x =0,2`(mol)
$*$ cho `Na_2CO_3` dư vào
`Ca(OH)_2` dư `= x + y+z-2x = y` (mol)
`Ca(OH)_2 + Na_2CO_3 -> CaCO_3↓ + 2NaOH`
`=> n↓ = y` (mol)`; m↓ =20` (g)
`=> y = (20)/(100)=0,2` (mol)
`=> m = mCa + mCaO +mCaC_2`
`=0,2.40+0,2.56 + 0,2.64=32` (g)
