flybry2000

10+ Year Member
5+ Year Member
Oct 21, 2008
89
2
91
Status
Pre-Dental
Hello SDNer's,

Got some Gen Chem Questions:

1) If a solution which is 0.5M in compund X decomposes for 5.0 minutes at an average rate of 0.040M/min, the new concentration of X will be:

a)0.04M
b)0.05M
c)0.2M
d)0.3M
e)0.5M

2) During a titration it was determined that
30.00 mL of a 0.100 M Ce4+ solution was
required to react completely with 20.00
mL of a 0.150 M Fe2+ solution. Which​
one of the following reactions occurred?
A. Ce4+ + 3Fe2+ + H2O 3Fe3+
+ CeO- + 2H+
B. 2Ce4+ + Fe2+ Fe4+ + 2Ce3+
C. Ce4+ + Fe2+ Fe3+ + Ce3+
D. Ce4+ + 2Fe2+ 2Fe3+ + Ce2+
E. Ce4+ + 2Fe2+ 2Fe4+ + Cs2+​
+ 2e-

3)
A 49-gram sample of sulfuric acid, H2SO4 (98 g mol-1), contains
A. 1 mol of S atoms.
B. 16.0 grams of O.
C. 2.0 grams of H.
D. 2 moles of O atoms.​
E. 1 mole of molecules.

1) d 2) c 3) d

Thanks
 

UndergradGuy7

10+ Year Member
Jun 23, 2007
897
2
241
Status
Pre-Dental
1 is 5 * 0.040 = 0.20 so 0.2M decomposes in 5 minutes. So 0.5 - 0.2 is 0.3 left.

2 if you find out the moles of Ce by doing 30 * 0.1 M = 3 mmol of Ce
and 20 * 0.150 = 3 mmol of Fe

This shows that the same amount of moles reacted or the mol ratio is 1:1.
The answer is C since the ratio between Ce4+ and Fe2+ is 1:1. Also notice the oxidation states of Fe and Ce change by 1 from reactants to products.

3 Since the 49 grams of H2SO4/ 98 grams/mol = 0.5 moles of H2SO4
in 0.5 moles of H2SO4 there is 0.5 moles S so A is wrong. There are 2 moles of O2 and 2 moles O2 = 32 g/mol, B is wrong. There is 0.5 mol H2 which is 1 gram, so C is wrong. E is wrong as we know we have 0.5 moles.

This is from the ADA sample exam isn't it?
 

flybry2000

10+ Year Member
5+ Year Member
Oct 21, 2008
89
2
91
Status
Pre-Dental
Awesome! Thanks.

The answers made more sense the 2nd time around. But I was wondering for the second problem when you multiplied the Molarity times the mL, shouldn't the mL be converted to Liters to cancel out the units? Eitherway the moles were eqaul to each other (i.e. I got 0.003mol for each).

This is something I've noticed for the M1V1=M2V2 problems. It just seems like everything should be changed to units that cancel out with each other.

Yeah, it's from the ADA sample test...if only the real thing was this easy:laugh:

Thanks again,

Flybry2000
 

UndergradGuy7

10+ Year Member
Jun 23, 2007
897
2
241
Status
Pre-Dental
If you leave the volume in ml then when you multiply by the molarity, you will get millimoles instead of moles (moles / 1000). You could convert the ml to liters and do the calculation, but why would you? That's a waste of time.

You could also just convert the final mmol to moles by dividing by 1000.