# Introductory Chemistry

□ Supplemental/Summer School □

Examinations of December

/April/May □ /July □ 2009

Originating Campus:

Cave Hill

□

Mona

St. Augustine □

Mode:

On Campus

By Distance □

Semester l

Semester II

Course Code and Title: CHEM1901 - INTRODUCTORY CHEMISTRY A

Date:

Duration:

Time:

2

Hours.

Paper No: 1

Materials required:

Answer booklet:

Calculator:

(where applicable)

Multiple Choice answer sheets:

Programmable

□

□

numerical

□

Normal

Special

Not required

□

Non Programmable

□ 1-20 □ 1-100 □

alphabetical

Auxiliary/Other material(s) – Please specify:

CHEMISTRY ANSWER BOOKLET

Candidates are permitted to bring the following items to their desks:

Instructions to Candidates: This paper has

4

pages &

6

questions.

Candidates are reminded that the examiners shall take into account the proper use of the English Language in determining the mark for each response.

This paper has three (3) sections. You are required to answer FOUR questions in total. Answer at least ONE question from EACH section.

Begin each question on a new page

The numbers in brackets on the right hand side of the paper are the marks allocated.

2

Section A

ATOMIC STRUCTURE

1. (a) The Lithium ion, Li2+, consists of a single electron moving about the Lithium nucleus. (i) Calculate the gravitational force, and the electrostatic force on the electron, which is at a distance of one-third the Bohr radius from the nucleus. ( a o = 0.5292 Å, gravitation constant: G = 6.672 x 10-11 N m2 kg-2) (8)

(ii) Comment on the magnitude and significance of the values obtained in (a)(i)

(4)

(b) The wavefunctions of hydrogen atoms, Ha and Hb, are ψa and ψb, respectively. (i) Given that ψa and ψb can be positive or negative, write expressions for the possible wavefunctions for the interaction of Ha and Hb.

(4)

(ii) Determine the expressions for the intensities in each case in (b)(i).

(4)

(iii)Comment, with the aid of diagrams, on the significance of the intensities obtained in part (b)(ii).

(5)

2. (a) Calculate the de Broglie wavelength of:

(i) an electron traveling at a velocity of 2.6 x 108 m s -1

(ii) a 50 g bullet traveling at 50 km hr-1.

Comment on the significance of the values obtained.

(3)

(3)

(3)

(b) The Heisenberg Uncertainty Principle (HUP) can be expressed as, ∆q.∆v ≥

h

4πm

(i) Define all the terms in the expression.

(ii) Explain, with reference to the expression, what you understand by the HUP.

(c) Define the three quantum numbers n, l and ml, and state what specific information about atomic orbitals is given by each.

(3)

(5)

(8)

3

Section B

ANALYTICAL CHEMISTRY

3. Answer ALL parts.

(a) Answer each of the following:

i) Distinguish between volumetric and gravimetric analytical methods. ii) Define buffer capacity.

(4)

(b) A solution was prepared by dissolving 1.572 grams of NaB (Relative Molecular Mass 169), the sodium salt of a weak base, in 100.0 cm3 of water. pKa for the weak acid HB is 6.3. Calculate the pH of the solution. Kw = 1 x 10-14.

(6)

(c) 20.0 cm3 of the basic solution is titrated with 0.134 mol dm-3 HCl. Calculate: i) the equivalence volume,

ii) the pH at the equivalence point,

iii) the pH at the point of half neutralization,

iv) the pH 5.0 cm3 after the equivalence point.

(1)

(4)

(3)

(2)

(d) Using your answers to parts (b) and (c) above as a guide, sketch the variation of pH with titrant volume and select a suitable indicator for the titration from the table, below, of indicators and their pKa values. Give a reason for your choice of indicator.

(5)

indicator

methyl yellow

bromocresol green

chlorophenol red

phenol red

thymol blue

thymolphthalein

4.

pKa

3.3

4.7

6.0

7.4

8.9

9.9

indicator

methyl orange

methyl red

bromothymol blue

cresol purple

phenolphthalein

alizarin yellow R

pKa

4.2

5.0

7.1

8.3

9.7

11.0...

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