Question

What massof Na2SO4 • 10H2O is needed to prepare a solution of Na2SO4(aq)
containing 1.5 × 10^20 anions? (Be sure to indicate the appropriate number of
significant figuresin the final answer.)

Answer 1

Na₂SO₄ dissociates to give 2 Na⁺ and one SO₄^- ions

So 1.5 * 10²⁰ anions means 1.5 * 10²⁰ SO₄^- ions which means it requires 1.5 * 10²⁰ molecules of Na₂SO₄

Number of moles of Na₂SO₄ = 1.5 *10²⁰ / avagadro number

                                                   = 1.5 * 10²⁰/ 6.022 * 10²³

                                                   = 0.24908 * 10^-3 moles of Na₂SO₄

One mole of Na₂SO₄ means 322.30 gm of Na₂SO₄

So 0.24908 * 10^-3 moles of Na₂SO₄ = 322.30 * 0.24908 * 10^-3

                                                                  =80.2785 * 10^-3

                                                                = 0.0803 gm of Na₂SO₄

Hence 0.0803 gm off Na₂SO₄ decahydrate is required to prepare solution containing 1.5 * 10²⁰ anions