To attack a complex problem, try a 'divide and conquer' strategy as follows:
Step 1: Choose your target. You want to get a percentage of iron in the ore. If you had the mass of iron in the ore and the mass of the ore, you'd have your answer. The mass of ore is given, but the mass of iron isn't: your target is g Fe.
Step 2: Examine the given information.
The problem gives you several pieces of information; look at each one closely and try to decide how it is related to your target.
|0.206 g KMnO4 in 100.0 mL KMnO4 solution||This can be used to relate moles of KMnO4 to L KMnO4 solution.|
|0.238 g iron ore in 50.0 mL distilled water||The amount of ore will be necessary to compute the percentage of the iron in the ore. (By the way, water won't work as a solvent here. I believe this part of the problem should specify that all of the iron in the ore is converted to Fe2+(aq)).|
|1.7 mL of H2SO4||This tells you the redox reaction between the KMnO4 and the Fe2+ is occuring in an acidic solution. The amount of H2SO4 is not crucial here, since the limiting reagent in the titration will be the Fe2+.|
|10.3 mL of KMnO4 solution at endpoint||This tells you how much KMnO4 is required to completely react with all of the Fe2+ from the ore sample. (KMnO4 is deep purple and all the other species in the titration are nearly colorless. Just one drop of excess permanganate imparts a definite pink tinge to the solution.)|
Step 3: Relate the target to information given in the problem.
The key piece in the remainder of the problem is relating the amount of KMnO4 added to the amount of iron in the ore:
mL of KMnO4 solution g Fe
Whenever you're confronted with a problem that asks for the amount of one substance given an amount of another substance, you must look for a mole-to-mole relationship between the two substances. Look for mole-to-mole relationships in chemical formulas or chemical equations supplied in the problem. In this case, the mole-to-mole relationship must be obtained from a balanced chemical equation between the Fe2+ and the KMnO4. I'll give you the half reactions involved here in aqueous solution and let you combine them into a balanced equation for the titration yourself:
MnO4-(aq) + 8H+(aq) + 5e- = Mn2+(aq) + 4H2O(l)
Fe2+(aq) = Fe3+(aq) + e-
Once you have the balanced equation, you can look at the coefficients and convert mol KMnO4 to mol Fe. This gives you a 'bridge' from your starting point to the target:
mL of KMnO4 solution
The mole-to-mole relationship cuts the problem up into 3 easier pieces:
- Milliliters of KMnO4 are converted to mol KMnO4 via the KMnO4 molarity that you've already calculated.
- mol KMnO4 is converted into mol Fe2+ via the mole-to-mole relationship you obtain from the balanced chemical equation.
- mol Fe2+ is converted into g Fe via the atomic weight of iron.
Step 4: Perform and check the calculation.Verify that all the units in the calculation we set up in the previous step 'cancel' consistently. You should get an iron mass that is some fraction of 0.238 g (the iron can't weigh more than the ore it came from, right?) Use the grams of iron and the grams of ore to obtain the percentage of iron in the ore.
Author: Fred Senese email@example.com