Submitted by akcook4 on Tue, 11/19/2013 - 18:07

A certain semimetal M emits electrons only when struck by radiation with a minimum frequency
of 9.07 x 1014 s-1. On heating a 0.200 g sample of the semimetal in the air, the corresponding
oxide M2O3 was obtained. When the oxide was dissolved in aqueous acid, H3MO3 was formed.
When this was titrated with KMnO4, 6.67 mL of 0.100 M MnO4- was required to reach an
equivalence point. The unbalanced equation is

H3MO3(aq) + MnO4-(aq) → H3MO4(aq) + Mn2+(aq) (in acid)

a. Balance the equation.
b. How many moles of semimetal M were in the initial 0.200 g sample?
c. What is the identity of the semimetal M?
d. What is the minimum energy (in kJ/mol) required to cause the semimetal to emit
electrons?

Please show all your work so that I can do future problems... THANK YOU!!!!!

Separate the reaction into oxidation and reduction half equations and balance in the usual way

MnO4- ---> Mn2+

MnO4- ---> Mn2+ + 4H2O (balance O by adding water)

MnO4- + 8H+  ---> Mn2+ + 4H2O (balance H by adding H+ ions)

MnO4- + 8H+  + 5e- ---> Mn2+ + 4H2O (reduction) (balance charge by adding electrons)

H3MO3(aq)  → H3MO4(aq)

H3MO3(aq) + H2O  → H3MO4(aq)

H3MO3(aq) + H2O  → H3MO4(aq) + 2H+

H3MO3(aq) + H2O  → H3MO4(aq) + 2H+ + 2e- (Oxidation)

Balance for electrons

2MnO4- + 16H+  + 10e- ---> 2Mn2+ + 8H2O (reduction)

5H3MO3(aq) + 5H2O  → 5H3MO4(aq) + 10H+ + 10e- (Oxidation)

2MnO4- + 16H+  + 10e- + 5H3MO3(aq) + 5H2O ---> 2Mn2+ + 8H2O + 5H3MO4(aq) + 10H+ + 10e-

2MnO4- + 6H+  + 5H3MO3(aq) ---> 2Mn2+ + 3H2O + 5H3MO4(aq)-

Then you know that 2 mole MnO4- react with 5 moles H3MO3

Find moles MnO4- from volume in L x molarity.

Then find moles H3MO3 from the 2:5 stoichiometry

Find moles M2O3 from M2O3 + 3H2O ----> 2H3MO3 knowing each mole M2O3 makes 2 moles H3MO3.

Then find moles M from 4M + 3O2 ---> 2M2O3

You then know moles of M and its mass (0.200g)

Find the mass of 1 mole of M and identify the element.

Submitted by kingchemist on Wed, 11/20/2013 - 09:41 Permalink