A few chemistry questions

I know the answer to the following multiple-choice questions (in bold) but don't know the reason.

1. Which salt would form a neutral solution when dissolved in water?

A. FeCl3
B. Na2CO3
C. KBr
D. NH4NO3

2. The mass of a metal deposited during electrolysis does not depend on

A. the current flowing.
B. the voltage between the electrodes.
C. the time for which the current passes.
D. the charge on the metal ion.

3. Which of the following atoms has/have one or more unpaired electrons?

I. Iron, II. Copper, III. Zinc

A. I only
B. III only
C. I and II only
D. I, II and III

4. The delocalisation of electrons is most likely to be significant in

A. CO2
B. SO2
C. HCOOH
D. TiO2

5. The shape of the triiodide, I3-, is best described as:

A. bent
B. linear
C. T-shaped
D. triangular

6. Which statement(s) about the following reaction at 100ºC is/are correct?

N2 (g) + 3H2 (g) <-> 2NH3 (g)

I. Every collision between N2 and H2 molecules is expected to produce NH3.
II. This reaction must involve a collision between one N2 and three H2 molecules.

A. I only
B. II only
C. Both I and II
D. Neither I nor II

(I know that I is false, but don't know why II is false.)

7. What are the [H+] and [OH-] in a 0.1 M solution of a weak acid (Ka = 1.0 * 10^-7)

[H+] = 1.0 * 10^-4
[OH-] = 1.0 * 10^-10

8. Which of the following combinations will form a buffer solution?

I. CH3COOH and CH3COONa
II. CH3COOH and NaOH

Both I and II

9. The mass spectrum of CH3COOC2H5 is not expected to show major peak at which m/e ratio?

A. 88
B. 32
C. 29
D. 15

Thanks in advance :smile:

Reply 1

nota bene

3 Copper obviously has unpaired as it has an uneven number of electrons. Iron has it since in the d-shell it has 6 e and the d-shell can have 10 e, and first of all the electrons fill each orbital, hence having 6e would mean 4 unpaired. Zink on the other hand has a filled d-shell i.e. all electrons are paired.

6 II is false because it can take place in steps, first the N2 collides with one H2 and then another one etc. In reality it is highly unlikely that N2 would collide with 3H2 at once.

This was those questions I felt I can answer at the moment... hopefully my reasoning is correct, otherwise I guess someone will correct me=)

Anyone?

2. The reaction that occurs depends on the electron transfer at the electrodes and so current (larger flow of electrons), more time (more time for electrons to flow) and charge (no of electrons vary depending on the charge of the metal) all have an effect on this whereas the voltage just needs to be great enough to allow the current to flow but once it is above this value it doesnt make any difference if it is increased.

9. In the MS peaks will occur at 88 which is the molecular ion peak. Then 29 is equal to the peak caused by the C2H5 ion and then 15 is caused by the CH3 ion.

Reply 4

hungry_hog

Answers in red

1. Which salt would form a neutral solution when dissolved in water?

A. FeCl3
B. Na2CO3
C. KBr
D. NH4NO3

think about the acidity / basicity of the parent compounds:

FeCl3 is from Fe(OH)3 + HCl. HCl strong acid Fe(OH)3 only moderate base thus the solution will be slightly acidic
Na2CO3 - we know is basic
KBr is from KOH (strong base) + HCl (strong acid) gives a neutral solution
NH4NO3 is from NH3 (moderate base) + HNO3 (strong acid) thus solution will be slightly acidic

2. The mass of a metal deposited during electrolysis does not depend on

A. the current flowing.
B. the voltage between the electrodes.
C. the time for which the current passes.
D. the charge on the metal ion.

the voltage is in effect provided by the redox reactions.

M(+) + e -> M and
NM(-) -> NM + e-

3. Which of the following atoms has/have one or more unpaired electrons?

I. Iron, II. Copper, III. Zinc

A. I only
B. III only
C. I and II only
D. I, II and III

answered above

4. The delocalisation of electrons is most likely to be significant in

A. CO2
B. SO2
C. HCOOH
D. TiO2

The sulfur is a larger atom and thus more polarizable than carbon, thus will get a greater contribution from resonance forms O(-)-S(+)=O and 0=S(+)-O(-) and greater delocalization.
Ti02 ruled out as it is giant covalent

5. The shape of the triiodide, I3-, is best described as:

A. bent
B. linear
C. T-shaped
D. triangular

the central I has 7 electrons, plus one from the - charge = 8
uses two of these to form two bonds to the terminal I atoms
that leaves 6e-. these are three lone pairs. to minimise repulsion they are arranged in a trigonal plane at 120 deg to each other. this leaves the terminal I atoms perpendicular to this i.e. all the I atoms in a line

6. Which statement(s) about the following reaction at 100ºC is/are correct?

N2 (g) + 3H2 (g) <-> 2NH3 (g)

I. Every collision between N2 and H2 molecules is expected to produce NH3.
II. This reaction must involve a collision between one N2 and three H2 molecules.

A. I only
B. II only
C. Both I and II
D. Neither I nor II

(I know that I is false, but don't know why II is false.)

answered above

7. What are the [H+] and [OH-] in a 0.1 M solution of a weak acid (Ka = 1.0 * 10^-7)

[H+] = 1.0 * 10^-4
[OH-] = 1.0 * 10^-10

use pka = -log ka this is bookwork!

8. Which of the following combinations will form a buffer solution?

I. CH3COOH and CH3COONa
II. CH3COOH and NaOH

Both I and II

don' understand the answer! would have thought I only

9. The mass spectrum of CH3COOC2H5 is not expected to show major peak at which m/e ratio?

A. 88
B. 32
C. 29
D. 15

can only get 32 from O2. but since the oxygens are not connected in the parent ion they will not show as O2 in the mass spec