Answer: 0.0015 M
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
