Sept. 22, 1993 NAME___________________________

Sept. 23/24, 1999 NAME____________________________

Chemistry 103 Section 1

EXAM #1--50 min.

IMPORTANT NOTE--For partial credit be sure to show your reasoning. Sometimes your instructor may go to heroic lengths to give you partial credit if you show work that is both legible logical. If you need more paper, write on the back of the page and indicate clearly that you have done so. Units and significant figures are important!

Look over all questions before starting and plan your time wisely.

There will be no partial credit on Problem 1.

(Possibly Useful )Physical Constants

NA = 6.022 x 1023

Mass of an electron 9.1 x 10-28 g

Mass of a proton 1.67 x 10-24 g

Mass of a neutron 1.67 x 10-24 g

Density of water 1.00 g/cm3

Solubility Table

Periodic Table

Problem 1________/20 10 basic 10 HW level

Problem 2________/20 10 basic 10 HW level

Problem 3________/20 20 HW level

Problem 4________/20 20 HW level

Problem 5________/20 20 synthetic level

Total _________

Score	grade (1sf)
90-100	4 Excellent, keep up the good work
70-90	3 Keep practicing--you are almost there
50-70	2 More and different studying may be needed
20-50	1 Serious revision of study strategies may be necessary
<20	0

Summary--Many of you did quite well and those who missed points often missed them on basic concepts (balancing reactions, molar masses, units--practice, practice, practice and make sure you get the correct numerical answer). Please see me if you would like to discuss your exam. If you did not do well here, please remember that I do look for improvement during the semester.
Chemistry majors invite you to a Chemical Tea--Sunday, Sept. 26, 9 P.M in the Chem. Lounge.

NAME____________________________

Problem 1 (20 points)

Indicate whether the quantity in Column A is greater than (>), equal to (=), or less than (<) the quantity in column B. You may write the words, the symbols or both. No partial credit will be awarded on the parts of this Problem. Part (a) is done for you as an example.

Column A Column B

a) The number of atoms in a mole	is greater than >	100 atoms

(b). The number of neutrons in a radioactive ³⁵S atom 35-16=19 neutrons	>	The number of neutrons in a radioactive ³²P atom. 32-15=17 neutrons

(c). The number of moles of the limiting reagent left after the reaction has gone to completion. 0	<	The number of moles of products present after the reaction has gone to completion.

(d) The number of elements classified as "metalloids".	<	The number of elements classified as metals.

(e) The oxidation state (O. S.) of O in H₂O₂. -1	>	The oxidation state (O. S.) of O in CO₂. -2

(f). The number of ions in a 1.0 M solution of CH₃COOH, a weak acid. incomplete ionization	<	The number of ions in a 1.0 M solution of HCl, a strong acid. complete ionization

NAME____________________________

Problem 2--Two or three sentences should be sufficient for each part. (20 pts)

a) To conduct top-quality chemical research it is important to have very pure water and many laboratories have a water purification system which monitors the electrical conductivity of the water. According to the glossy advertising pamphlets, the less the water is able to conduct electricity, the more pure it is.

Explain briefly how this method works and whether it is suitable for detecting all possible water impurities.

Ions are needed to conduct electricity so strong and weak electrolyte impurities will be detected. However, impurities which are neutral, molecular compounds, such as glucose, will be be detected because they do not ionize.

Water does not ionize under these conditions. Insoluble compounds are generally removed by filtration or allowed to settle to the top or bottom of the container and thus do not reach the purification system.

Comment on one aspect of the development of Atomic theory in the early 1800's. Describe an important experiment and how the results supported the newly developing atomic theory and disproved the existing belief.

Rutherford's experiment disproved the "plum pudding" model which postulated that the atom was of uniform consistency. He shot alpha particles, composed of 2 protons and 2 neutrons, at a thin layer of gold foil. Many particles passed straight through, but some were strongly deflected. Because of these strong deflections Rutherford advanced the nuclear model of the atom whereby each atom has a very small, but dense nucleus. Electrons circulate in the largely empty atomic periphery. Only alpha paricles which hit the nucleus directly were strongly deflected while those passing through the "empty" area in which the electrons circulate were not deflected.

NAME____________________________

Problem 3 (20 points)

It is the summer of 2000 and with the aid of the Bryn Mawr Alumnae network you have landed a wonderful job at a Biotech company engaged in gene therapy research. You are doing some DNA experiments that are extremely salt sensitive and a detailed reading of the label on a small vial of DNA says "sodium salt of deoxyribonucleic acid". You know that 10.0 mg (3 s.f.) of DNA were dissolved in 1.50 mL (3 s.f.) deionized, pure water and that DNA is a polyanion composed of repeating phosphate/sugar/base units. Each phosphate/sugar/base unit has a charge of -1 and a mass of 330 g/mol (2 s.f.).

What is the concentration of sodium ions?

HINT--What must be the overall electrical charge of the solution?

There must be as many Na+ ions as DNA has - charges. For each 330 g/mol DNA has one - charge.

Strategy: Calculate the concentration of - charges on DNA.

10.0 mg x (1 mol - charges /330 g) x (1/1.50 mL) = 0.020 M - charges on DNA

so there must be 0.020 M sodium ions.

NAME____________________________

Problem 4 (20 points)--Most did well here but balancing, inverted yields, and molar mass caluculations were trouble spots.

What volume of ethanol, C₂H₆O, is produced from the fermentation of 200 grams (3 s.f.) of glucose, C₆H₁₂O₆? The density of ethanol is 0.785 g/cm³ (3 s. f.) and the yield of the reaction below is 55% (2 s.f).

C₆H₁₂O₆(aq) ------>2 C₂H₆O (aq) + 2 CO₂(g)

Strategy

Balance Equation

Find # moles of glucose using M = 180 g/mol

Use 2 moles ethanol/1 mole glucose

Find # moles of ethanol

Find mass of ethanol using M = 46 g/mol

Find volume ethanol using density

Find actual volume using 55% yield.

200 g (1 mol/180 g) (2 moles ethanol/1mole glucose)(46 g /mol)(1cm³/.785g)(55/100) =

72 cm³ (2 s.f.)

Problem 5 (20 pts)

A mad scientist grabs two stock solutions. The first is 1.10 M KCl and the second is 3.75 M Ag₂(SO₄). Because silver is expensive, she mixes 5.00 mL of the silver stock solution with 45.0 mL of water then adds 50.0 mL of the potassium chloride solution to the mix while stirring. She carefully filters the resulting solution, dries the filter, and then weighs it.

Write balanced chemical equations to describe what has just taken place.

This is a double replacement reaction. Water does not react.

2K⁺ +2Cl^- + 2Ag⁺ + SO₄^2- ----> 2AgCl (s) + 2K⁺+ SO₄^2-

net ionic equation

Cl^- + Ag⁺ ----> AgCl (s) This insoluble remains on the filter paper.

By how much (in grams or mg) has the mass of the filter paper changed?

This is a limiting reagent problem.

moles Ag₂SO₄ = 5 mL x (3.75 mol/liter) = 0.01875 moles or 0.0375 moles Ag+

moles KCl 50 mL x (1.1 mol / liter) = 0.55 moles (excess--silver is limiting)

0.0375 moles AgCl (143.5 g/mol) = 5.38 g left on filter paper.