Chem question(molecular speed, oxidation state).. what I do wrong?

[NeoPTLD]

I.

What is the RMS speed of molecular O2 in ambient air(20C) at 1atm?

Vrms= sqrt of (3RT/M)

R=constant(8.314J/K*mol)
T=temp in kelvin
M=molecular weight in kg.(O2=16g/mol or 0.016kg/mol)

(3*8.314*293/0.016kg)=7308/0.016=456750.

*sqrt 456750=675.83m/s* my answer
the correct answer is apparently 478m/s
*never mind...* I should have used 32g/mol since O2 is 32g/mol where as O is 16g/mol




What is the oxidation state of each of the elements in this compound?

Al2(SO4)3, aluminum sulfate

SO4^- is always -2, and there are three which gives total negative charge of -6, therefore each aluminum must be +3.

Al=+3
O= -2
S=+8

since there are four oxygens each w/ a charge of -2 comgined to sulfur, I think sulfur has to be +8, but the key says +6...

 

[DXM]

I can't help you find the RMS velocity of a molecule as I kind of skipped that part of class. But for the oxidation state of Sulfur, check your math again, I got an oxidation state of +6.

Solution:

Total ionic charge = 0,

Al = +3
O = -2
S = ?

Total Al = 2*(+3) = +6
Total O = 12*(-2) = -24
Total S = 3*(X)

So (+6) + (-24) + 3x = 0
3X = 18
X = 6

Just have to keep your numbers straight.

 

[NeoPTLD]

Originally posted by: DXM
I can't help you find the RMS velocity of a molecule as I kind of skipped that part of class. But for the oxidation state of Sulfur, check your math again, I got an oxidation state of +6.

Solution:

Total ionic charge = 0,

Al = +3
O = -2
S = ?

Total Al = 2*(+3) = +6
Total O = 12*(-2) = -24
Total S = 3*(X)

So (+6) + (-24) + 3x = 0
3X = 18
X = 6

Just have to keep your numbers straight.

I thought I was supposed to split up the atoms in polyatomic ions the last.

SO4 or sulfate has a charge of 2+. Since there are three of these stuck to aluminum, it adds up to 6+
There are two aluminums and a charge of 6-, so each one must be 3-

Now, going back to SO4..

O= 2- and that's a given. There are four and four times 2- = 8-
to balance out with 8-, S would have to equal 8+.

 

[DXM]

I thought I was supposed to split up the atoms in polyatomic ions the last.

SO4 or sulfate has a charge of 2+. Since there are three of these stuck to aluminum, it adds up to 6+
There are two aluminums and a charge of 6-, so each one must be 3-

Now, going back to SO4..

O= 2- and that's a given. There are four and four times 2- = 8-
to balance out with 8-, S would have to equal 8+.

Whichever way you do the problem, you get sulfur with an oxidation state of +6. SO4 has an ionic charge of (-2) not (+2).

 

[NeoPTLD]

Originally posted by: DXM

I thought I was supposed to split up the atoms in polyatomic ions the last.

SO4 or sulfate has a charge of 2+. Since there are three of these stuck to aluminum, it adds up to 6+
There are two aluminums and a charge of 6-, so each one must be 3-

Now, going back to SO4..

O= 2- and that's a given. There are four and four times 2- = 8-
to balance out with 8-, S would have to equal 8+.

Whichever way you do the problem, you get sulfur with an oxidation state of +6. SO4 has an ionic charge of (-2) not (+2).

good catch... i still can't see how you get 6 though..

 

[DXM]

Personally, I think the first way I demonstrated is easier to express algebraically but if your teacher/professor wants you to examine the oxidation states of individual ions here's how to do it.

Total ionic charge of SO4 = -2

Given: Oxygen always has an oxidation state of -2 (except for peroxides)

So
1*(Oxidation state of sulfur) + 4*(-2) = (-2)
(Oxidation state of sulfur + (-8) = (-2)
(Oxidation state of sulfur) = +6