Question

Calculate the molarity of s2o32− if 3.0 ml 0.10 m k2s2o8, 2.0 ml 0.10 m k2so4, 1.0 ml 2% starch, 2.0 ml 0.010 m na2s2o3, and 5.0 ml 0.20 m ki are mixed.

Answer 1

Calculate the moles of S2O3 you have, that is the concentration of Na2S2O3 * the volume in L

Add up all the volumes, convert to L

Divide moles by the volume in L.

Answer 2

Answer: 0.0015 M

Step-by-step explanation:

First, rewrite the species, amounts and the concentrations for better understanding:

• S₂O₃²⁻
• 3.0 ml 0.10 M (molar) K₂S₂O₈
• 2.0 mol 0.10 m K₂SO₄
• 1.0 ml 2% starch,
• 2.0 ml 0.010 M (molar) Na₂S₂O₃
• 5.0 ml 0.20 M (molar) KI

Now you must use two basic assumptions:

• There is no chemical reaction among the different compounds of the mixture.

• Na₂S₂O₃ dissociates 100% as per:

Na₂S₂O₃ → 2Na⁺ + S₂O₃²⁻

That means that all the S₂O₃²⁻ comes from the 2.0 ml 0.010 M (molar) Na₂S₂O₃, and every mol of Na₂S₂O₃ yields 1 mol of S₂O₃²⁻.

And follow these steps:

1) Number of moles of S₂O₃²⁻

a) Molarity formula: M = n / V (in liters)

b) Clear n: n = M × V (in liters)

c) Substitute the numbers: n = 0.01 M × 2.0 ml × 0.001 liter/ml = 0.00002 moles

2) Volume of solution:

a) Add the volumes of all the solutions given:

V = 3.0 ml + 2.0 ml + 1.0 ml + 2.0 ml + 5.0 ml = 13.0 ml

b) Convert to liters: 13.0 ml / 1000 (ml / liter) = 0.013 liter

c) Calculate M (molarity)

M = n / V (in liter) = 0.00002 moles / 0.013 liter = 0.0015 M