An unknown solution containing one of the saltsZnSo4,Mgso4er SrCl2 gives a negat

Question

An unknown solution containing one of the saltsZnSo4,Mgso4er SrCl2 gives a negative flame test but tarnishes a piece of magnesium inserted into the solution. Which salt was in the solution? The heating of Cuso4. 5H2Os yields several transitions as a function oftemperature. The table below gives the percentage of original mass as function approximate temperature. e these percentages to determine the sequence of decompositions and write chemical equations for each transition. 14.5 29.0 36.0 51.5 67.0 70.5 percentage of original mass lost 100 130 220 800 910 1020 approximate temperature (°C) c 2015 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole orin part

Explanation / Answer

Answer 1)

1) As per "Reactivity series" of metals we have the increasing trend for metals (corresponding to give salts) as,

(least reactive) Zn < Mg < Sr (Most reactive).

This is the trend in which tendency of metal to lose electrons increases. Sr is easiest to lose electrons and Zn is hardest in same.

In other words we can understand this trend is that,

In presence of Mg metal it is the Zn2+ cation which readily accepts electrons from Mg metal and not the Sr2+ ions.

(because Sr is easier to go into Sr2+ state which means back electron gaining is hard for Sr2+).

In given experiment, It;s observed that unknown solution tarnishes Mg meatal. which involve the reaction as,

Mg (s) ---------> Mg2+ (aq) + 2e-.

(Tarnishing means removal of superfacial lyer of metal).

Aqueos solutions of salts used are, ZnSO4 (aq), MgSO4 (aq), SrSO4 (aq).

Which being strong electrolytes ionizes as,

ZnSO4 (s) ---------> Zn2+ (aq) + SO4-2 (aq)

MgSO4  (s) ---------> Mg2+ (aq) + SO4-2 (aq)

SrSO4  (s) ----------> Sr2+ (aq) + SO4-2 (aq).

As per above discussion it is the Zn2+ which can readilly (not only preferrentially but the only species) which force Mgo to lose electrons to get itself reduced Zno.

Zn2+ (aq) + Mgo ---------> Zno + Mg2+ (aq).

Answer :Hence unknown solution is ZnSO4 (aq).

Flame test at support : It is Zn2+ only which gives colourles flame i.e. apparent -ve flame test.

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