Central Chemistry

Showing posts with label General Chemistry. Show all posts
Showing posts with label General Chemistry. Show all posts

Sunday, January 2, 2022

Chapter 1 — Pratice Test : Matter (Its Properties and Measurement)

Chapter 1 — Pratice Test : Matter (Its Properties and Measurement)

Question 1

The volume of seawater on Earth is about 330,000,000 m3 If seawater is 3.5% sodium chloride by mass and has a density of 1.03 g/mL what is the approximate mass of sodium chloride, in tons, dissolved in the seawater on Earth (1 ton = 2000 lb)

Question 2

The diameter of metal wire is often referred to by its American wire-gauge number. A 16-gauge wire has a diameter of 0.05082 in. What length of wire, in meters, is found in a 1.00 lb spool of 16-gauge copper wire? The density of copper is 8.92 g/cm3.

Question 3

A typical rate of deposit of dust (“dustfall”) from unpolluted air was reported as 10 tons per square mile per month. (a) Express this dustfall in milligrams per square meter per hour. (b) If the dust has an average density of 2 g/cm3, how long would it take to accumulate a layer of dust 1 mm thick?

Monday, December 27, 2021

Chapter 1 — Matter (Its Properties and Measurement)

Chapter 1 — Matter (Its Properties and Measurement)

From the clinic that treats chemical dependency to a theatrical perfor- mance with good chemistry to the food label stating “no chemicals added,” chemistry and chemicals seem an integral part of life, even if everyday references to them are often misleading. A label implying the absence of chemicals in a food makes no sense. All foods consist entirely of chemicals, even if organically grown. In fact, all material objects— whether living or inanimate—are made up only of chemicals, and we should begin our study with that thought clearly in mind. By manipulating materials in their environment, people have always practiced chemistry. Among the earliest applications were glazing pottery, smelting ores to produce metals, tanning hides, dyeing fabrics, and making cheese, wine, beer, and soap. With modern knowledge, though, chemists can decompose matter into its smallest components (atoms) and reassemble those components into materials that do not exist naturally and that often exhibit unusual properties. Thus, motor fuels and thousands of chemicals used in the manufacture of plastics, synthetic fabrics, pharmaceuticals, and pesticides can all be made from petroleum. Modern chemical knowledge is also needed to understand the processes that sustain life and to understand and control processes that are detrimental to the environment, such as the formation of smog and the destruction of stratospheric ozone. Because it relates to so many areas of human endeavor, chemistry is sometimes called the central science.

Early chemical knowledge consisted of the “how to” of chemistry, discovered through trial and error. Modern chemical knowledge answers the “why” as well as the “how to” of chemical change. It is grounded in principles and theory, and mastering the principles of chemistry requires a systematic approach to the subject. Scientific progress depends on the way scientists do their work—asking the right questions, designing the right experiments to supply the answers, and formulating plausible explanations of their findings. We begin with a closer look into the scientific method.

1-1 The Scientific Method

—The scientific method is a set of procedures used to develop explanations of natural phenomena and possibly to predict additional phenomena. The four basic stages of the scientific method are (1) gathering data through observations and experiments; (2) reducing the data to simple verbal or mathematical expressions known as natural laws; (3) offering a plausible explanation of the data through a hypothesis; (4) testing the hypothesis through predictions and further experimentation, leading ultimately to a conceptual model called a theory that explains the hypothesis, often together with other related hypotheses.

1-2 Properties of Matter

—Matter is defined as anything that occupies space, possesses mass, and displays inertia. Composition refers to the component parts of a sample of matter and their relative proportions. Properties are the qualities or attributes that distinguish one sample of matter from another. Properties of matter can be grouped into two broad categories: physical and chemical. Matter can undergo two types of changes: chemical changes or reactions are changes in composition; physical changes are changes in state or physical form and do not affect composition.

1-3 Classification of Matter

—The basic building blocks of matter are called atoms. Matter that is composed of a collection of a single type of atom is known as an element. A sample of matter composed of two or more elements is known as a compound. A molecule is the smallest entity of a compound having the same proportions of the constituent atoms as does the compound as a whole. Collectively, elements and compounds compose the types of matter called substances. Mixtures of substances can be classified as homogeneous or heterogeneous (Fig. 1-4). The three states of matter are solid, liquid, and gas.

Wednesday, October 6, 2021

Diluting Concentrated Solutions

Diluting Concentrated Solutions

For convenience, chemicals are sometimes bought and stored as concentrated solutions, which are then diluted before use. Aqueous hydrochloric acid, for example, is sold commercially as a 12.0 M solution, yet it is most commonly used in the laboratory after dilution with water to a final concentration of either 6.0 M or 1.0 M.

Concentrated solution Solvent → Dilute solution


The main thing to remember when diluting a concentrated solution is that the number of moles of solute is constant; only the volume of the solution is changed by adding more solvent. Because the number of moles of solute can be calculated by multiplying molarity times volume, we can set up the following equation:


where Mi is the initial molarity, Vi is the initial volume, Mf is the final molarity, and Vf is the final volume after dilution. Rearranging this equation into a more useful form shows that the molar concentration after dilution (Mf) can be found by multiplying the initial concentration (Mi) by the ratio of initial and final volumes (Vi > Vf):

Monday, February 4, 2019

Precipitation Reactions in Aqueous Solution

Precipitation Reactions in Aqueous Solution

One common type of reaction that occurs in aqueous solution is the precipitation reaction, which results in the formation of an insoluble product, or precipitate. A precipitate is an insoluble solid that separates from the solution. Precipitation reactions usually involve ionic compounds. For example, when an aqueous solution of lead(II) nitrate [Pb(NO3)2] is added to an aqueous solution of potassium iodide (KI), a yellow precipitate of lead iodide (PbI2) is  formed:

Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)

Potassium nitrate remains in solution. Figure 1 shows this reaction in progress. The preceding reaction is an example of a metathesis reaction (also called a double displacement reaction), a reaction that involves the exchange of parts between two compounds. (In this case, the compounds exchange the NO3- and I- ions.) As we will see, the precipitation reactions discussed in this chapter are examples of metathesis reactions.

Figure 1 Formation of yellow PbI2 precipitate as a solution of Pb(NO3)2 is added to a solution of KI.

How can we predict whether a precipitate will form when a compound is added to a solution or when two solutions are mixed? It depends on the solubility of the solute, which is defned as the maximum amount of solute that will dissolve in a given quantity of solvent at a specif  c temperature. Chemists refer to substances as soluble, slightly soluble, or insoluble in a qualitative sense. A substance is said to be soluble if a fair amount of it visibly dissolves when added to water. If not, the substance is described as slightly soluble or insoluble. All ionic compounds are strong electrolytes, but they are not equally soluble. Table 1 classif es a number of common ionic compounds as soluble or insoluble. Keep in mind, however, that even insoluble compounds dissolve to a certain extent. Figure 2 shows several precipitates.

Table 1 Solubility Rules for Common Ionic Compounds in Water at 25˚C

Molecular Equations, Ionic Equations, and Net Ionic Equations
The equation describing the precipitation of lead iodide on page 100 is called a molecular equation because the formulas of the compounds are written as though all species existed as molecules or whole units. A molecular equation is useful because it identifes the reagents (that is, lead nitrate and potassium iodide). If we wanted to bring about this reaction in the laboratory, we would use the molecular equation. However, a molecular equation does not describe in detail what actually is happening in solution.

Figure 2 Appearance of several precipitates. From left to right: CdS, PbS, Ni(OH)2, Al(OH)3.

As pointed out earlier, when ionic compounds dissolve in water, they break apart into their component cations and anions. To be more realistic, the equations should show the dissociation of dissolved ionic compounds into ions. Therefore, returning to the reaction between potassium iodide and lead nitrate, we would write

Sunday, February 3, 2019

Explanation About Electrolytes versus Nonelectrolytes

Explanation About Electrolytes versus Nonelectrolytes

All solutes that dissolve in water fit into one of two categories: electrolytes and nonelectrolytes. An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity. A nonelectrolyte does not conduct electricity when dissolved in water. Figure 1 shows an easy and straightforward method of distinguishing between electrolytes and nonelectrolytes. A pair of platinum electrodes is immersed in a beaker of water. To light the bulb, electric current must flow from one electrode to the other, thus completing the circuit. Pure water is a very poor conductor of electricity. However, if we add a small amount of sodium chloride (NaCl), the bulb will glow as soon as the salt dissolves in the water. Solid NaCl, an ionic compound, breaks up into Na+ and Cl- ions when it dissolves in water. The Na+ ions are attracted to the negative electrode and the Cl- ions to the positive electrode. This movement sets up an electrical current that is equivalent to the flow of electrons along a metal wire. Because the NaCl solution conducts electricity, we say that NaCl is an electrolyte. Pure water contains very few ions, so it cannot conduct electricity. Comparing the lightbulb’s brightness for the same molar amounts of dissolved substances helps us distinguish between strong and weak electrolytes. A characteristic of strong electrolytes is that the solute is assumed to be 100 percent dissociated into ions in solution. (By dissociation we mean the breaking up of the compound into cations and anions.) Thus, we can represent sodium chloride dissolving in water as