Hello guys ,
I'm sitting for the chemistry exam this year and I have started studying for it. The spectator ions have being bothering me a lot recently. In the books I'm doing they neglect them sometimes and write ionic equations and do all sorts of stuff with it. I know specatator ions aren't involved in the reaction but I get confused with them sometimes.I'll post some multiple choice problems here to make it clear where my difficulties lie. The answers are also provided. So all I really want is to understand how to get the answer.
Problem 1
A standard solution of pottassium permenganate (KMn04) has a concentration of 0.0240 M. It is titrated against a solution of iron (II) sulfate. The equation fo the reaction is. Sorry I don't know how to put numbers underneath.
5Fe2+ + Mn04- + 8H+ = 5Fe3+ + Mn2+ + 4H20
15.60 ml of the KMn04 solution reacts exactly with 20 ml of the FeSO4 solution. The concentration of the FeSO4 solution, in M, is
The answer in the back of the book is 0.0936
My problem- K and S04 are not even in the equation of the question. I think it is because they are spectator ions. Then the only concentration I can find is the concentration of Fe2+. How is this the same as the concentration of FeS04
Problem 2
50 ml of 0.020 M solution of Ba(OH)2 is added to 50 ml of a 0.060 M solution of HNO3. The hydrogen ion concentration in the resulatant solution, in mole per litre, is
Answer in the back of the book: 0.01 M
My problem: Normally I work out the excess reactant in a question like this and see the moles in excess. But when I work out if Ba(0H)2 or HNO3 is in excess. I don't work out H+ ions. Can I get the moles of H+ ions in excess because it is equal to moles of HNO3 in excess. Does that mean if the reactant is H2NO3 ( I just made this up) instead of HNO3. I times by 2 to get the H+ ions in the resulatnat solution. Another problem I have is acid and base produce H20. Why don't we add the H+ ions in the H20 to the solution
Problem 3
20.0 ml of 0.10 M HCL reacts with 20 ml of 0.30 M KOH solution. The concentration of pottassium ions in the resultant solution, in mole per litre ,is
The answer in the back of the book- 0.15. They are saying that K+are spectator ions, hence n(K+) does not change. However the volume changes to 40 ml hence the concentration is halved.
My problem: Normally I find the excess and limiting reactant in a question like this. Then I look at the moles in excess and divide that by the total volume. Why is that moles of spectator ions do not change.
I don't expect you to understand everything I said. I tried my best to explain. If you can help me with these questions or pass any tips when working with spectator ion questions I'll very much appreaciate it
. Thanks!!