[Xenonman]

Hello, all 
My class is covering complexometry currently. EDTA complexes actually.
To "help us", the teacher gave an exercise I have problems with. The thing is, we were given a formula for solving them without thinking a bit. I tried a slightly more analytical way, which gives me the same base number, with a different order of magnitude. I asked the teacher about this difference, who claimed my method wrong because I calculated the Y^-4 moles, which is "impossible" because the concentration is given in normality, not molarity.

All the exercises ask hardness of water in ppm of CaCO₃.

The brainless method is: ppm=1000*N_Y-4*V_{Y-4, ml}/V_{sample, ml}
My method multiplies the Y^-4 volume with the normal or molar concentration (reaction is 1:1, so N=M) to obtain n_Y^-4, which are equal to n_Ca^+2 and allegedly equal to n_CaCO3. Said moles of CaCO3 are converted to milligrams, then divided by the sample volume in liters. This yields mg/L, also known as ppm.

I searched some other exercises and solved them with my and the other method. Again, both answers have the same base, with different orders of magnitude. My method answers what the other people who posted the other exercises, the other one doesn't:
The one I have problems with: 50 ml of a water sample are titrated with 43.6 ml of a 0.1 N EDTA solution. What is the hardness of water in ppm? My answer: 8720 ppm, using the other method I get 87.2 ppm
The other ones:
50 ml of a water sample are titrated with 25.55 ml of a 0.0149 N EDTA solution.   My answer: 761.39 ppm, the other method gives 7.6139 ppm. The answer the guy who uploaded it gave is 762 ppm.
35 ml of a water sample are titrated with 9.7 ml of a 0.01 M EDTA solution. My answer is 277.14 ppm, the other method gives 2.77. The answer the guy who uploaded it gave is 277 ppm.

As you can guess, I'm not really sure of what I am supposed to do.

[Borek]

The brainless method is: ppm=1000*N_Y-4*V_{Y-4, ml}/V_{sample, ml}

As you have mentioned ppm is typically mg/L, so this formula is wrong, as it doesn't convert from number of moles to mass.  By sheer accident, molar mass of CaCO₃ is almost exactly 100 g, so it looks like all answers are off just by orders of magnitude, but for other substances you would get results looking completely off.

Your approach is perfectly correct. Nothing wrong with finding number of moles of EDTA, as its normality equals molarity in chelation (complexation) reactions. Besides, you can always use number of moles and follow exact stoichiometry of teh reaction, this is even better than just blindly using ready "formulas".

[Xenonman]

I asked with another teacher. He said I should use the analytical approach, showing the calculations step by step.
In case I get a big X after answering the test, ask the first teacher to show where the error is, instead of me challenging the previously shown formula.  This plan B is fail proof in my eyes. Is it on anyone else's?

[Borek]

Depends on the teacher. Some have no problems when you point out they are wrong, some will seek revenge.