Calculate the mass of-
(a) 0.2 mol molecules of oxygen
(b) one atom of aluminium
(c) 3.0 mol of Chloride ion
(d) 2.5 mol of NaCl. (Atomic masses: `O = 16u, Cl = 35.5 u, Na = 23 u, Al = 27 u`)

Explanation

(a)Molecular mass of oxygen `(O_(2) = 32 u)`
Molar mass of `O_(2) = 32 g mol^(-1)`
Now, 1 mole of `O_(2)` molecule = 32 g mole
`therefore O_(2)` molecule `= 3.2 xx 0.2 = 6.4 g`
(b) Atomic mass (Al) = 27 u
`therefore` Molar mass `=27 g "mol"^(-1)`
Now `6.022 xx 10^(23)` atoms of aluminium weigh = 27g
`therefore` 1 atom of aluminium will weigh `= 27/(6.022 xx 10^(23)) = 4.48 xx 10^(-23) g`
(c) Formula mass of CI ion = 35.5 u
`therefore` Molar mass of CI ions = `35.5 g mol^(-1)`
Number of moles = 3 (given)
Now, `n =("m"("Mass"))/("M"("Molar mass"))`
`rArr m = n xx M = 3 xx 35.5 = 106.5 g`
(d) Formula mass of NaCl = 23 + 35.5 = 58.5 u
`therefore` Molar mass of NaCl (M) = `58.5 g "mol"^(-1)`
Number of moles = 2.5 (given)
Now, `n =m/M`
`rArr m = n xx M`
` = 2.5 xx 58.5 = 146.25 g`