Establishing Empirical and Molecular Formulas 08.10.2013

Consider the following 5 step approach to determine formulas:

1. Choose an arbitrary sample size (100 g).

2. Convert masses of the elements in 100 g sample to amounts in moles.

3. Write a tentative formula based on the #s of moles just determined.

4. Divide each of the subscripts by the smallest one.

5. If all subscripts differ only slightly from whole #s, then, round them off. Otherwise multiply all subscripts by a small whole number to make the subscripts integral.

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Solution Dilution 22.10.2013

In general, it is not practical to store solutions of every possible concentration. Instead, we generally store fairly concentrated solutions, so-called stock solutions.

Dilute solutions, then, can be prepared from stock solutions. A concentrated solution has a relatively large amount of dissolved solute.

A dilute solution has a small amount.

When a solution diluted, the amount of solute remains constant between the initial (i) and final (f) solution prepared.

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Combining the Gas Laws: (The Ideal Gas Equation and the General Gas Equation) The ideal gas equation is a single equation that includes all four gas variables; V, T, P, amount of gas (n). Any gas that obeys this equation is said to be an ideal gas (or perfect gas).

Example: What is the volume occupied by 13.7 g Cl2 at 45⁰C and 745 mmHg ?

Solution:

Example: How many molecules of N2 remained in an ultrahigh vacuum system of 128 mL volume when the pressure is reduced to 5×10-10 mmHg at 25⁰C ? (R=0.0821 L.atm.mol-1.K-1, NA = 6.02×1023)

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The part of the universe we choose to study is called a system. The parts of the universe with which the system interacts are called the surroundings. Three systems: open system, closed system and isolated system.

Internal Energy (E) is the total amount of energy (potential, kinetic, chemical energy,…) contained in a system. The components of internal energy are:

Translational kinetic energy

Molecular rotation.

Bond vibration.

Intermolecular attractions.

Chemical bonds.

Energy is the capacity to do work.

Work is done when a force acts through a distance. A system contains only internal energy.

– A system does not contain heat (q) or work (w).

– Energy transfer can occur as heat or work during a change in the system.

Chemical reactions may also do work.

• Gas formed pushes against the atmosphere.

• Volume changes.

• Pressure-volume work is done.

The most common type of work associated with chemical reactions is pressure-volume work. This is work involved in the expansion or compression of gases.

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Boiling and the Boiling Point When a liquid is heated in a container open to atmosphere, vapor bubbles form within the bulk of the liquid, rise to the surface throughout the liquid and escape. When the pressure exerted by escaping molecules equals that exerted by the gases of atmosphere, The heat absorbed during boiling is used only to convert liquid molecules to vapor. The temperature at which the vapor pressure of a liquid is equal to Standard atmospheric pressure (1 atm = 760 mmHg) is the normal boiling point. the boiling occurs.

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Critical Point and Supercritical Fluids (SCF)

If a liquid is heated in a sealed container, boiling doesn’t occur. The temperature and vapor pressure rise continuously many times the atmospheric pressure. Then, we observe the followings;

1) density of liquid decreases; that of vapor increases and eventually the two densities become equal,

2) surface tension of the liquid approaches zero. The meniscus between the liquid and vapor becomes less distinct and eventually disappears. The fluid is not neither a liquid nor a gas now.

The critical point is the point at which these conditions are reached and the liquid and vapor become indistinguishable. At this point, the temperature is the critical temperature (Tc) and the pressure is the critical pressure (Pc).

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Solutions are homogeneous mixture of 2 or more substances (Chp 4).

– Uniform throughout.

• Solvent.

– Determines the state of matter in which the solution exists.

– Is the largest component.

– Dissolves solute

• Solute.

– Other solution components said to be dissolved in the solution.

A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature.

An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature.

An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity.

A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity.

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Ionic Solutions

e.g., NaCl in H2O

1. Ions that are less tightly held because of their position at a corner or an edge of the crystal are exposed to water molecules

2. Water molecules will collide with the NaCl until an ion breaks free

3. More water molecules then cluster around the ion, stabilizing it by ion dipole attractions

4. The water molecules attack the weak part of the crystal until it is dissolved

5. Ions in solution are said to be solvated (hydrated) they are surrounded and stabilized by an ordered shell of solvent (water) molecules

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