[Twoacross]

I am truly sorry for the double post, i should have known "help" is not good in the title:


So i just did a gravimetric experiment where we took an unknown mass of a compound containing nickel and with the data gathered and some formulas to work with,

          Ni^2+ (aq) + 2 C4H7O2N2H (alc) =====>  Ni (C4H7O2N2)2 (s) + 2H^+ (aq)

Let me list my data first:

Mass of original sample: 1.0601 g

Mass of precipitate

9A: 0.2857 g
9B: 0.2815 g

And im asked to find the percentage of nickel in the unknown sample, which would be the 1.0601 g

So what i did first was took the mass of 9A's precipitate and converted it to moles,
    0.2857 g / 288.9158 g /mol = 9.8887 x 10^-4 mol Ni(C4H7O2N2)2
From here, i converted that value into moles of nickel which would be the same since:

       9.8887 x 10^-4 mol Ni(C4..)2 x 1 mol Ni divided by 1 mol of Ni(C4..)2 = 9.8887 x 10^-4 mol Ni

From here i can calculate the mass by taking the mass of the nickel and multiplying it by the molar mass of nickel:

       9.8887 x 10^-4 mol Ni * 58.693 g / mol = 0.058039 g Ni

And then i divided it by the unknown sample's mass to obtain:

      0.05803 g / 1.0601 g *100% = ~5.50%

2. Then the second part asks which of these choices my sample could be:

      NiCl2*6H2O,   NiSO4*6H2O,    NiSO4*7H20,     NiBr2*6H2O

Now what i decided to do was compare the percentages of nickel within each compound, I thought of a couple ways of doing this:

1. Take the molar mass of nickel and divide it by the total molar mass of the given compounds
2. Find equations like taking NiCl2*6H2O and then adding the alc shown above and then from there balance it and then here i thought which mass i would use, the unknown or the precipitate?

I gave it some thought and im still kinda stump on which approach would be appropriate

All feedback and help is greatly appreciated!

Thank you!

[fledarmus]

Compare:

And then i divided it [the mass of Ni] by the unknown sample's mass [the total mass of the compound] to obtain:

      0.05803 g / 1.0601 g *100% = ~5.50%

to

1. Take the molar mass of nickel and divide it by the total molar mass of the given compounds

Does this help answer your question?

[Twoacross]

Hmm, I tried doing that but none of the compounds to choose from are not close to 5%, do I go with the one with the nearest percent?

Thanks

[Borek]

You have two results - does it mean you did two identical experiments, starting with two identical 1.0601g samples?

[Twoacross]

Okay, its clear now, Thanks everyone, I guess I overcomplicate things whereas I should be thinking more theoretically.

Thanks Borek and fledarmus =D