Molar Mass of An Element and Avogadro's Number

This page will describes molar mass of an element and avogadro's number.

Atomic mass units provide a relative scale for the masses of the elements. But since atoms have such small masses, no usable scale can be devised to weigh them in calibrated units of atomic mass units. In any real situation, we deal with macroscopic samples containing enormous numbers of atoms. Therefore it is convenient to have a special unit to describe a very large number of atoms. The idea of a unit to denote a particular number of objects is not new. For example, the pair (2 items), the dozen (12 items), and the gross (144 items) are all familiar units. Chemists measure atoms and molecules in moles.

In the SI system the mole (mol) is the amount of a substance that contains as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12 grams (or 0.012 kilogram) of the carbon-12 isotope. The actual number of atoms in 12 g of carbon-12 is determined experimentally. The currently accepted value is

1 mole = 6.022045 x 10²³ particles

This number is called Avogadro’s number, in honor of the Italian scientist Amedeo Avogadro. Generally, we round Avogadro’s number to 6.022 x 10²³. Thus, just as one dozen oranges contains twelve oranges, 1  mole of hydrogen atoms contains 6.022 x 10²³ H atoms. (The Chemistry in Three Dimensions essay on p. 106 describes a method for determining Avogadro’s number.)

We have seen that 1 mole of carbon-12 atoms has a mass of exactly 12 grams and contains 6.022 x 10²³ atoms. This mass of carbon-12 is its molar mass, defined as the mass (in grams or kilograms) of 1 mole of units (such as atoms or molecules) of a substance. Note that the molar mass of carbon-12 (in grams) is numerically equal to its atomic mass in amu. Likewise, the atomic mass of sodium (Na) is 22.99 amu and its molar mass is 22.99 grams; the atomic mass of phosphorus is 30.97 amu and its molar mass is 30.97 grams; and so on. If we know the atomic mass of an element, we also know its molar mass. Using atomic mass and molar mass, we can calculate the mass in grams of a single carbon-12 atom. From our discussion we know that 1 mole of carbon-12 atoms weighs exactly 12 grams. This allows us to write the equality

12.00 g carbon-12 = 1 mol carbon-12 atoms

Therefore, we can write the unit factor as

(12.00 g carbon-12) / (1 mol carbon-12 atoms)  =1

(Note that we use the unit “mol” to represent “mole” in calculations.) Similarly, since there are 6.022 x 10²³ atoms in 1 mole of carbon-12 atoms, we have

1 mol carbon-12 atoms 6.022 x 10²³ carbon-12 atoms 

and the unit factor is

(1 mol carbon-12 atoms) / (6.022 x 10²³  carbon-12 atoms) = 1

Atomic Mass

This page describes about chemical structure and formulas in studying the mass relationships of atoms and molecules. These relationships in turn will help us to explain the composition of compounds and the ways in which
composition changes.

The mass of an atom depends on the number of electrons, protons, and neutrons it contains. Knowledge of an atom’s mass is important in laboratory work. But atoms are extremely small particles—even the smallest speck of dust that our unaided eyes can detect contains as many as 1 1016 atoms! Clearly we cannot weigh a single atom, but it is possible to determine the mass of one atom relative to another experimentally. The first step is to assign a value to the mass of one atom of a given element so that it can be used as a standard.

By international agreement, atomic mass (sometimes called atomic weight) is the mass of the atom in atomic mass units (amu). One atomic mass unit is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom. Carbon-12 is the carbon isotope that has six protons and six neutrons. Setting the atomic mass of carbon- 12 at 12 amu provides the standard for measuring the atomic mass of the other elements. For example, experiments have shown that, on average, a hydrogen atom is only 8.400 percent as massive as the carbon-12 atom. Thus, if the mass of one carbon-12 atom is exactly 12 amu, the atomic mass of hydrogen must be 0.0084 12.00 amu or 1.008 amu. Similar calculations show that the atomic mass of oxygen is 16.00 amu and that of iron is 55.85 amu. Thus, although we do not know just how much an  verage iron atom’s mass is, we know that it is approximately fifty-six times as massive as a hydrogen atom.

Average Atomic Mass

When you look up the atomic mass of carbon in a table such as the one on the inside front cover of this book, you will find that its value is not 12.00 amu but 12.01 amu. The reason for the difference is that most naturally occurring elements (including carbon) have more than one isotope. This means that when we measure the atomic mass of an element, we must generally settle for the average mass of the naturally occurring mixture of isotopes. For example, the natural abundances of carbon-12 and carbon- 13 are 98.90 percent and 1.10 percent, respectively. The atomic mass of carbon- 13 has been determined to be 13.00335 amu. Thus the average atomic mass of carbon can be calculated as follows:

average atomic mass of natural carbon = (0.9890)(12.00000 amu) + (0.0110)(13.00335 amu)
= 12.0 amu

A more accurate determination gives the atomic mass of carbon as 12.01 amu. Note that in calculations involving percentages, we need to convert percentages to fractions. For example, 98.90 percent becomes 98.90/100, or 0.9890. Because there are many more carbon-12 atoms than carbon-13 atoms in naturally occurring carbon, the average atomic mass is much closer to 12 amu than to 13 amu. It is important to understand that when we say that the atomic mass of carbon is 12.01 amu, we are referring to the average value. If carbon atoms could be examined individually, we would find either an atom of atomic mass 12.00000 amu or one of 3.00335 amu, but never one of 12.01 amu. The following example shows how to calculate the average atomic mass of an element.

Chemical and Physical Properties of Matter

This page describes chemical and physical properties of matter. As we know, every matter has different properties.

Substances are identified by their properties as well as by their composition. Color, melting point, and boiling point are physical properties. A physical property can be measured and observed without changing the composition or identity of a substance.
For example, we can measure the melting point of ice by heating a block of ice and recording the temperature at which the ice is converted to water. Water differs from ice only in appearance, not in composition, so this is a physical change; we can freeze
the water to recover the original ice. Therefore, the melting point of a substance is a physical property. Similarly, when we say that helium gas is lighter than air, we are referring to a physical property.

On the other hand, the statement “Hydrogen gas burns in oxygen gas to form water" describes a chemical property of hydrogen, because in order to observe this property we must carry out a chemical change, in this case burning. After the change, the original chemical substance, the hydrogen gas, will have vanished, and all that will be left is a different chemical substance—water. We cannot recover the hydrogen from the water by means of a physical change, such as boiling or freezing. Every time we hard-boil an egg, we bring about a chemical change. When subjected to a temperature of about 100 C, the yolk and the egg white undergo changes that alter not only their physical appearance but their chemical makeup as well. When eaten, the egg is changed again, by substances in our bodies called enzymes. This digestive action is another example of a chemical change. What happens during digestion depends on the chemical properties of both the enzymes and the food.

All measurable properties of matter fall into one of two additional categories: extensive properties and intensive properties. The measured value of an extensive property depends on how much matter is being considered. Mass, which is the quantity of
matter in a given sample of a substance, is an extensive property. More matter means more mass. Values of the same extensive property can be added together. For example, two copper pennies will have a combined mass that is the sum of the masses of each penny, and the length of two tennis courts is the sum of the lengths of each tennis court. Volume, defined as length cubed, is another extensive property. The value of an extensive quantity depends on the amount of matter.

The measured value of an intensive property does not depend on how much matter is being considered. Density, defined as the mass of an object divided by its volume, is an intensive property. So is temperature. Suppose that we have two beakers of water at the same temperature. If we combine them to make a single quantity of water in a larger beaker, the temperature of the larger quantity of water will be the same as it was in two separate beakers. Unlike mass, length, and volume, temperature and other intensive properties are not additive.

sp Hybridization of Alkynes

The common hybridization in alkynes is sp. Alkynes are compound that have triple bond between it's carbons.

Acetylene is linear, with a carbon–carbon bond distance of 120 pm and carbon–hydrogen bond distances of 106 pm.

Linear geometries characterize the H-C≡C-C and C-C≡C-C units of terminal and internal triple bonds, respectively as well. This linear geometry is responsible for the relatively small number of known cycloalkynes. Figure below shows a molecular model for cyclononyne in which the bending of the C-C≡C-C unit is clearly evident. Angle strain destabilizes cycloalkynes to the extent that cyclononyne is the smallest one that is stable enough to be stored for long periods. The next smaller one, cyclooctyne, has been isolated, but is relatively reactive and polymerizes on standing. In spite of the fact that few cycloalkynes occur naturally, they gained recent attention when it was discovered that some of them hold promise as anticancer drugs.

All of these trends can be accommodated by the orbital hybridization model. The bond angles are characteristic for the sp3, sp2, and sp hybridization states of carbon and don’t require additional comment. The bond distances, bond strengths, and acidities are related to the s character in the orbitals used for bonding. s Character is a simple concept, being nothing more than the percentage of the hybrid orbital contributed by an s orbital. Thus, an sp3 orbital has one quarter s character and three quarters p, an sp2 orbital has one third s and two thirds p, and an sp orbital one half s and one half p. We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and p contributors. Take C-H bond distance and bond strength, for example. Recalling that an electron in a 2s orbital is, on average, closer to the nucleus and more strongly held than an electron in a 2p orbital, it follows that an electron in an orbital with more s character will be closer to the nucleus and more strongly held than an electron in an orbital with less s character. Thus, when an sp orbital of carbon overlaps with a hydrogen 1s orbital
to give a C-H bond, the electrons are held more strongly and the bond is stronger and shorter than electrons in a bond between hydrogen and sp2-hybridized carbon. Similar reasoning holds for the shorter C-C bond distance of acetylene compared to ethylene, although here the additional π bond in acetylene is also a factor.

The pattern is repeated in higher alkynes as shown when comparing propyne and propene. The bonds to the sp-hybridized carbons of propyne are shorter than the corresponding bonds to the sp2 hybridized carbons of propene.

Nomenclature of Alkynes

Alkynes can be named both in IUPAC and trivial methods. Here's the nomenclature of alkynes.

The systematic name of an alkyne is obtained by replacing the “ane” ending of the alkane name with “yne.” Analogous to the way compounds with other functional groups are named, the longest continuous chain containing the carbon–carbon triple bond is numbered in the direction that gives the alkyne functional group suffix as low a number as possible. If the triple bond is at the end of the chain, the alkyne is classified as a terminal alkyne. Alkynes with triple bonds located elsewhere along the chain are called nternal alkynes. For example, 1-butyne is a terminal alkyne, whereas 2-pentyne is an internal alkyne.

In common nomenclature, alkynes are named as substituted acetylenes. The common name is obtained by citing the names of the alkyl groups, in alphabetical order, that have replaced the hydrogens of acetylene. Acetylene is an unfortunate common name for the smallest alkyne because its “ene” ending is characteristic of a double bond rather than a triple bond. If the same number for the alkyne functional group suffix is obtained counting from either direction along the carbon chain, the correct systematic name is the one that contains the lowest substituent number. If the compound contains more than one substituent, the substituents are listed in alphabetical order.

Source of Alkynes

Alkynes are useful chemical. They're can be intermediate for other organic chemical's synthesis. They're so many sources of alkynes.

Acetylene was first characterized by the French chemist P. E. M. Berthelot in 1862 and did not command much attention until its large-scale preparation from calcium carbide in the last decade of the nineteenth century stimulated interest in industrial applications. In the first stage of that synthesis, limestone and coke, a material rich in elemental carbon obtained from coal, are heated in an electric furnace to form calcium carbide.

Calcium carbide is the calcium salt of the doubly negative carbide ion. Carbide dianion is strongly basic and reacts with water to form acetylene:

Beginning in the middle of the twentieth century, alternative methods of acetylene production became practical. One of these is based on the dehydrogenation of ethylene.

The reaction is endothermic, and the equilibrium favors ethylene at low temperatures but shifts to favor acetylene above 1150°C. Indeed, at very high temperatures most hydrocarbons, even methane, are converted to acetylene. Acetylene has value not only by itself but is also the starting material from which higher alkynes are prepared. Natural products that contain carbon–carbon triple bonds are numerous. Two examples are tariric acid, from the seed fat of a Guatemalan plant, and cicutoxin, a poisonous substance isolated from water hemlock.

Diacetylene (HC≡C-C≡CH) has been identified as a component of the hydrocarbon-rich atmospheres of Uranus, Neptune, and Pluto. It is also present in the atmospheres of Titan and Triton, satellites of Saturn and Neptune, respectively.