Answer to Question #25695 in Chemistry for Kathryn

Question

1) A 0.15 L solution of .30 M MgCl2 is mixed with .15 L of .10 M Na2CO3. Calculate the molar concentration of the carbonate ion left after MgCO3 precipitates. What percentage of the carbonate ion precipitated?

2)a)What is the concentration of SO4 just as BaF2 begins to precipitate when 1.0 M Ba(NO3)2 is added to a solution containing .60 M F and .50 M So4? b)What percent of the sulfate ion remains in solution when the BaF2 begins to preciptiate? c)O Would fractional precipitation work to separate BaF2 and BaSO4? EXPLAIN your answer.

Answer 1

1)
A 0.15 L_solution of 0.30 M MgCl₂ is mixed with 0.15 L of 0.10 M Na₂CO₃.
Calculate the molar concentration of the carbonate ion left after MgCO₃ precipitates. What percentage of the carbonate ion precipitated?
The chemical equation for this reaction is
MgCl₂(aq) + Na₂CO₃(aq) = 2NaCl(aq) + MgCO₃(s)
C = n/V
c - concentration (M)
n - amount (moles)
V – volume (L)
n(MgCl₂) = C(MgCl₂) x V(MgCl₂) = 0.30 x 0.15 = 0.045 mol
n(Na₂CO₃) = C(Na₂CO₃) x V (Na₂CO₃) = 0.10 x 0.15 = 0.015 mol
According to the chemical equation:
n(MgCl₂) = n(Na₂CO₃) = 0.015 mol and it’s 0.030 mol of MgCl₂ is left in solution after reaction is completed.
Answer: After MgCO₃ precipitates it’s no carbonate ion left in the solution. The percentage of the carbonate ion precipitated 100%

2)
a) What is the concentration of SO₄ just as BaF₂ begins to precipitate when 1.0 M Ba(NO₃)₂ is added to a solution containing 0.60 M F and 0.50 M SO₄^2-?
b) What percent of the sulfate ion remains in solution when the BaF₂ begins
to precipitate?
c) Would fractional precipitation work to separate BaF₂ and BaSO₄? EXPLAIN your answer.

Answer:
The ion equation for these reactionsare
Ba²⁺(aq) + SO₄^2-(aq) = BaSO₄(s)
Ba²⁺(aq) + 2F^-(aq) = BaF₂(s)
From the equations we can see that the amount of Ba²⁺ is more than it’s needed to complete both of the reactions:
n(Ba²⁺) = n(SO₄^2-) + n(F^-)/2 = 0.50 + 0.60/2 = 0.50 + 0.30 = 0.80 mol, but we have 1.0 mol of Ba²⁺ ions in the solution.
That’s why if we pour 1 M solution of Ba²⁺ at once, both of the substances will precipitate almost simultaneously, and it’s very hard to find out when exactly BaF2 starts to precipitate. But it is possible to separate BaF₂ and BaSO₄ with fractional precipitation, when we are adding a small amount of Ba²⁺ gradually. As solubility products of BaSO₄ and BaF₂ differ in 10^4 times
[K_s(BaSO₄) = 1.1 x 10^-10; K_s(BaF₂) = 1.1 x 10^-6], BaSO₄ will precipitate first, it can be collected, and only after this next adding of Ba²⁺ ions leads to precipitation of BaF₂.