I've a few quick chem questions if someone could help me out with them that'd be great..?

Question

Balance the following equation for a reaction in acidic solution. Only H+ or H2O may be added. Enter your answer as the sum of the coefficients. 
HgS(s) + Cl-(aq) + NO3- --> HgCl42-(aq) + NO2(g) + S(s)

The following reaction occurs in basic solution. Balance it by adding only OH- or H2O. Enter your answer as the sum of the coefficients. 
Mn2+(aq) + MnO4-(aq) --> MnO2(s)

Oxygen gas can be generated by heating KClO3 in the presence of some MnO2.
2KClO3 --> 2KCl + 3O2
If all the KClO3 in a mixture containing 2.31 g KClO3 and 0.35 g MnO2 is decomposed, how many liters of O2 gas will be generated at T = 21.0oC and P = 758.8 torr?

Christophe G · Accepted Answer

HgS + 4Cl- + 2NO3- + 4H+ -> HgCl4(2-) + 2NO2 + S + 2H2O

For the second one you need to write half equations
Mn is 2+ in Mn2+, 4+ in MnO2 and 7+ in MnO4-.

going from Mn2+ to MnO2 will require loss of 2 electrons so:
Mn2+ + 4OH-  -->  MnO2 + 2H2O + 2e-

going from MnO4- top MnO2 will require gain of 3 electrons so

MnO4- + 4H+ + 3e-  ---> MnO2 + 2H2O
(if you want to write it under basic conditions you would get MnO4- + 4H2O + 3e- ---> MnO2 + 2H2O + 4OH- but I will  only worry about that at the very end)

you need to put the equations together and the electrons must cancel out so you must multiply the first equation by 3 and the seacond y 2 and you get;

3Mn2+ + 12OH- + 2MnO- + 18H+  ----> 3MnO2 + 6H2O + 2MnO2 + 4H2O

combine things together and you get:


3Mn2+ + 2MnO4- +8 H2O + 4OH-   --->  5MnO2  + 10H2O

or 3Mn2+ + 2MnO4- + 4OH- ---> 5MnO2 + 2H2O

Even though we used one half equation under acidic condition, the final equation is still basic so no adjustment is necessary.

2 moles of KClO3 will make 3 moles of oxygen.

Find the molecualar weight of KClO3, use it with the mass to find how many noles of KClO3 were decomposed. multiply by 3/2 to find the number of moles of oxygen formed and use:
V = nRT/P to find the volume.

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I've a few quick chem questions if someone could help me out with them that'd be great..?

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