A few tough G-Chem problems

[topdent1]

I was taking a practice gen chem exam from my class a couple years ago, and came across a few questions I just couldn't figure out. Take a look...

1. A 151 g sample of a metal, M, reacts with exactly .35 moles of oxygen to form a metal oxide with the formula M2O. What is the identity of M?
a. Cs
b. Na
c. Au
d. Rb
e. Ag

2. How many electrons in a given atom can be described using the following quantum numbers? n=4, l=2, ml=2
a. 2
b. 3
c. 5
d. 6
e. 10

3. Determine the ground state electron configuration for Cr3+.
a. [Ar]4s2,3d1
b. [Ar]
c. [Ar]3d3
d. [Ar]4s2,3d7
e. [Ar]3d9

4. Is XeF2 a polar molecule? (I was just curious about this one)

Answers:
1. e
2. a
3. c

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[mddang]

1. A 151 g sample of a metal, M, reacts with exactly .35 moles of oxygen to form a metal oxide with the formula M2O. What is the identity of M?
a. Cs
b. Na
c. Au
d. Rb
e. Ag

4M + O2 -> M2O
If there are .35 moles of O2, then 1.4 moles of the metal are necessary to react to completion.

We have 151g of the metal which equals .7moles.
molar mass of mystery metal = 151g/1.4moles = 108g/mol
The metal w/ this molar mas is silver.

2. How many electrons in a given atom can be described using the following quantum numbers? n=4, l=2, ml=2
a. 2
b. 3
c. 5
d. 6
e. 10

n is the principal quantum number. It tells us which shell the electron is in.

l tells us which subshell (s, p, d, or f) the electron is in. The value of l ranges from 0 to n-1.

ml tells us which orbital.

Only two electrons at most can occupy any given orbital.

The answer is 2.

3. Determine the ground state electron configuration for Cr3+.
a. [Ar]4s2,3d1
b. [Ar]
c. [Ar]3d3
d. [Ar]4s2,3d7
e. [Ar]3d9

The electron configuration for metallic Cr is [Ar] 4s1, 3d5

Cr3+ has 3 less electrons. First, we lose the one electron from the 4s subshell, then two more from the 3d subshell are lost.

This gives is c. [Ar]3d3

4. Is XeF2 a polar molecule? (I was just curious about this one)

You have to draw the lewis dot structure and know a little about the VSEPR theory for this one.

We have three lone pairs on Xe and two atoms bonded to it.

The three lone pairs are going to be located on the same plane, and the two F atoms are going to be positioned as far away from each other as possible.

The molecule is linear. The angle between the two F molecules is 180 degrees, and the angle between one of the F atoms and the lone pairs is 90 degrees.

XeF2 is nonpolar. The two F molecules point in opposite directions, so the dipoles cancel out, which means no net dipole. It's like having two people who are equally strong (or weak) playing tug-of-war.

 

[DentalDeac]

Ok for #1, you know that you will need .70 mol of the metal M to react with the .35 mol Oxygen to form M2O. Does the problem say that you are making 2 mol of M2O or just one mole? When I solved, I did 151 g of M / .70 mol of M to give 215 g/mol as the molar mass of M. But if you divide this by 2 you get 107 which is Ag. But I'm not really sure where I missed something. Anyone?

 

[DentalDeac]

4M + O2 -> M2O
If there are .35 moles of O2, then 1.4 moles of the metal are necessary to react to completion.

We have 151g of the metal which equals .7moles.
molar mass of mystery metal = 151g/1.4moles = 108g/mol
The metal w/ this molar mas is silver.


n is the principal quantum number. It tells us which shell the electron is in.

l tells us which subshell (s, p, d, or f) the electron is in. The value of l ranges from 0 to n-1.

ml tells us which orbital.

Only two electrons at most can occupy any given orbital.

The answer is 2.

Nice work on number 1. I didn't take into account that the oxygen was diatomic, and therefore you would need 4M.

 

[Orgodox]

XeF2 is nonplolar even though both of the bonds are polar they cancel out since it is symetrical and has dipole moment of O