2. The concentration of Sr 2+ in seawater is 7.8 ppm (part per million or mg/kg)

Question

2. The concentration of Sr2+ in seawater is 7.8 ppm (part per million or mg/kg). The concentration of SO42- is 2710 ppm. The activity coefficients of Sr2+ and SO42- are 0.230 and 0.180 respectively.

a)Write the balanced reaction for the solubility of celestite (SrSO4)

b)Write the expression for the equilibrium constant for this reaction assuming conventional standard states.

c)Determine the change in entropy (S) and the change in enthalpy (H) for this reaction at 25°C.

d) What is the change in free energy (G) for this reaction at 20°C?

e)What assumptions did you make to determine G above? Are those assumptions valid (reasonable) in this case?

f)What is the value of the equilibrium constant for this reaction at 20

g)What is an ion activity product (IAP)? How does it differ from the equilibrium constant?

h)Evaluate the ion activity product (IAP) for celestite in seawater.

i)What is a saturation index? What does it tell you?

j)What is the saturation index of celestite at 20°C? Is celestite saturated, supersaturated or undersaturated in the shallow waters of the ocean?

Explanation / Answer

a) SrSO4 ----------->Sr2+ + SO4-2

b) k = [Sr]+2 [SO4]-2 / [SrSO4]

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