Question

Consider 18.06x10²⁴ molecules of Na₂S₄O₆ a) Number of moles of sulfur is? b) If dissolved to make 500.0 mL of solution the molarity will be?

Consider P₄ + 5O₄ → 2P₄O₅, in which 20.0g of oxygen are burned. c) The mass of P₂O₅ is?

Answer 1

(a) We know that 1 mole of any compound contains 6.023x10²³ ( avagadro number ) of molecules

So number of moles of Na₂S₄O₆ , n = (18.06x10²⁴ )/(6.023x10²³)

= 30 moles

1 mole of Na₂S₄O₆ contains 4 moles of Sulphur

30 moles of Na₂S₄O₆ contains 4x30 = 120 moles of Sulphur

(b) Molarity , M = number of moles / volume in L

= 30 mol / (500.0 mL x 10^-3 L/mL)

= 60.0 M

(c)   P₄ + 5O₄ → 2P₄O₅

Molar mass(g/mol) 124 204

From the balanced reaction,

1 mole = 124 g of P₄ produces 2 mol = 2x204 = 408 g of P₄O₅

20.0 g of P₄ produces M g of P₄O₅

M = ( 20.0 x 408) / 124

= 65.8 g of  P₄O₅