Chemistry

Using the following information, identify the strong electrolyte
whose general formula is

Mx(A)y * zH2O


Ignore the effect of interionic attractions in the solution.

a. A"-is a common oxyanion. When 30.0 mg of the anhydrous
sodium salt containing this oxyanion (Na,,A, where n = 1, 2,
or 3) is reduced, 15.26 mL of 0.02313 M reducing agent is
required to react completely with the Na,A present. Assume
a 1: 1 mole ratio in the reaction.

b. The cation is derived from a silvery white metal that is
relatively expensive. The metal itself crystallizes in a
- body-centered cubic'unit cell and has an atomic radius of
198.4 pm. The solid, pure metal has a density of 5.243 g/cm3.
The oxidation number of M in the strong electrolyte in question
is +3.

c. When 33.45 mg of the compound is present (dissolved) in
10.0 mL of aqueous solution at 25"C, the solution has an
osmotic pressure of 558 tor.

In section a above, "athe sodium salt containing this oxyanion(Na,,A, where n = 1,2, or 3)" where is the n? Is that NanA or NaAn?

It's NanA

Let me help you with A and let you work on the remainder yourself.
An oxyanion limits the anion; it must be one of the following or something similar.
phosphate
phosphite
sulfate
sulfite
bromate
iodate
chlorate
chlorite
hypochlorite
nitrate
nitrite
carbonate
The Na1,2,or 3A reduces with
15.26 mL of 0.02313 reucing agent.
0.01526 L x 0.02313 M = 0.00035296 mols.
A 1:1 reaction means 0.00035296 mols A^-. Then molar mass = g/mols = 0.030 g (from the problem)/0.00035926 = 84.995.
You can work through the scenarios like this.
84.995-3(Na) = 84.995 - (3x22.9898) = 16.026. That is what is left for a central atom + oxygen. If we subtract 1 oxygen from that (15.9994), there is left only about 0.02 for the central atom and that can't be. No central atom weighs that little. So Na3A is out.

Try two Na atoms (remember NanA where n = 1, 2, or 3.

84.995 - 2(22.9898) = 39.0154 left for the central atom + oxygens. Subtracting oxygen atoms:
less 1 O atom leaves 23.016
less 2 O atoms leaves 7.0166
and neither 23.016 nor 7.0166 are near the masses of I, Br, Cl, S, P, C, or N (see my quick summary below).

So we go to 1 Na.
84.995 - 22.9898 = 62.005 left for central atom + oxygen atoms.
less 1 O atom = 46.006
less 2 O atoms = 30.006
less 3 O atoms = 14.007

You can see from the quick summary below that 46 isn't close to any of those elements. 30 is relatively close to 30.97 for phosphorus. That would make the sodium salt NaPO2 and I don't know of a PO2 ion. Phosphate is PO4^-3 (already excluded) and phosphite is PO3^-3 (already excluded) so we scratch that one off, too, because the PO2^- isn't common and I don't think it exists. That leaves ONLY one, and N on the table is 14.0067 which is VERY close analytically to 14.007. So N probably is the central atom of the anion and it probably has three O atoms attached. That makes it the nitrate ion, NO3^- and the sodium salt is NaNO3. Check the molar mass of NaNO3, I get 84.9947 which rounds to 84.995 which can't get much closer to the analytically calculated value.

Just for reference,
I = about 127
Br = about 80
Cl = about 35.5
S = about 32
C = about 12
N = about 14
P = about 31


I hope this gets you started.


Check my numbers carefully. I may have rounded here or there with 1 number off (that's done easily) but the final result won't change. Here is a site to obtain the molar mass and atomic masses.
http://environmentalchemistry.com/yogi/reference/molar.html

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