Answer:
I would go for number 5 . It sounds more logical to me.
2) Increasing the rate of the reverse reaction will cause a shift to the left.
Hopefully, If it is wrong my full apologies.
OR IF THATS WRONG, TRY THIS
a - Increasing the rate of the forward reaction will cause equilibrium to be more product-favored (i.e. shift to the right, not the left)
if we did not use an excess of the BaCl2 solution it would decrease the mass percentage of sulfate in the unknown sample.
The net precipitation equation would be.
Ba2+(aq) + SO42-(aq) → BaSO4(s)
If BaCl2 (Ba2+) is not taken in excess then the precipitation would not be completed as some of the sulfate ions would still be remaining in the solution. This would decrease the mass percentage of sulfate in the unknown sample.
If some tiny pieces of filter paper still remained mixed with the precipitate(BaSO4) then the mass of sulfate would increase and it gives a high mass percentage of the sulfate.
mass percentage of sulfate = (mass of sulfate/mass of sample)*100
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