Which volume of 4.00 x 10-2 mol/L solution of barium nitrate, Ba(NO3)2(aq), contains
2.00 x 10-2 mol of nitrate ions?

0.25 L Barium nitrate solution contains 0.02 mol nitrate ions

Given

0.04 M Barium nitrate, Ba(NO₃)₂(aq)

Required

The volume of Barium nitrate

Solution

Ionization of Barium nitrate in 1 L solution :

Ba(NO₃)₂ ⇒ Ba²⁺ + 2NO₃²⁻

mol :

0.04             0.04    0.08

There is 0.08 mol of nitrate ions in 1 L solution

So for 0.02 mol nitrate ions :

= 0.02/0.08 x 1 L

= 0.25 L

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)