The Representative Elements are those elements within the first two families Groups I and II on the far left and the last six families or groups on the right of the Periodic Table. The Transition Metals are the elements in those Groups within the middle of the Table. Why is oxygen a diatomic molecule? Oxygen generally exists as a diatomic molecule in the atmosphere when it is not combined with any other element.
It forms the molecule O2 because in that configuration, it has its lowest energy level when uncombined. All substances tend to go to the lowest energy level possible. Oxygen has 6 valence electrons. What is the smallest unit representative of water in this system? An atom is the smallest unit of matter that retains all of the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids.
For example, water is composed of hydrogen and oxygen atoms that have combined to form water molecules. What is the SI unit of a substance? The International System of Units SI defines the amount of substance to be proportional to the number of entities present. The SI unit for amount of substance is the mole. It has the unit symbol mol. How do you find the number of particles? Key Concepts 1 mole of any substance contains 6.
To find the number of particles, N, in a substance: To find the amount of substance in moles, n :. How do you calculate number of atoms? To calculate the number of atoms in a sample, divide its weight in grams by the amu atomic mass from the periodic table, then multiply the result by Avogadro's number: 6. What is g mol? A gram mole is equal to the mass of one mole of substance Avogadro's Number 6.
Most chemists abbreviate mole and moles to mol and mols. What are the three representative particles? Matter is composed of three types of representative particles: atoms, molecules and formula units. How do you find the number of particles? Key Concepts 1 mole of any substance contains 6. To find the number of particles, N, in a substance: To find the amount of substance in moles, n :.
What is the SI unit of a substance? The International System of Units SI defines the amount of substance to be proportional to the number of entities present. The SI unit for amount of substance is the mole.
It has the unit symbol mol. Why is oxygen a diatomic molecule? Oxygen generally exists as a diatomic molecule in the atmosphere when it is not combined with any other element. It forms the molecule O2 because in that configuration, it has its lowest energy level when uncombined.
All substances tend to go to the lowest energy level possible. Oxygen has 6 valence electrons. Molar mass is equal to density multiplied by molar volume. How do I get from here to there? If I want to visit the town of Manteo, North Carolina, out on the coast, I will need a map of how to get there. I may have a printed map or I may download directions from the internet, but I need something to get me going in the right direction. Chemistry road maps serve the same purpose. How do I handle a certain type of calculation?
There is a process and a set of directions to help. Previously, we saw how the conversions between mass and number of particles required two steps, with moles as the intermediate. This concept can now be extended to also include gas volume at STP.
The resulting diagram is referred to as a mole road map see Figure below. The mole road map shows the conversion factors needed to interconvert between mass, number of particles, and volume of a gas.
The mole is at the center of any calculation involving amount of a substance. The sample problem below is one of many different problems that can be solved using the mole road map.
What is the volume of The given mass of neon is equal to about 4 moles, resulting in a volume that is about 4 times larger than molar volume.
Use the link below to carry out some practice calculations. Do problems 1, 2, and 5 you can try the others if you are feeling especially brave :. Is there anything healthy in this jar? Packaged foods that you eat typically have nutritional information provided on the label. The label on a jar of peanut butter shown above reveals that one serving size is considered to be 32 g.
The label also gives the masses of various types of compounds that are present in each serving. One serving contains 7 g of protein, 15 g of fat, and 3 g of sugar. By calculating the fraction of protein, fat, or sugar in one serving of size of peanut butter and converting to percent values, we can determine the composition of the peanut butter on a percent by mass basis.
Chemists often need to know what elements are present in a compound and in what percentage. The percent composition is the percent by mass of each element in a compound. It is calculated in a similar way that we just indicated for the peanut butter.
The sample problem below shows the calculation of the percent composition of a compound based on mass data. A certain newly synthesized compound is known to contain the elements zinc and oxygen. When a Determine the percent composition of the compound. Subtract to find the mass of oxygen in the compound. By mass, the compound is mostly zinc. The percent composition of a compound can also be determined from the formula of the compound.
The subscripts in the formula are first used to calculate the mass of each element in one mole of the compound. The percent composition of a given compound is always the same as long as the compound is pure. Dichlorineheptoxide Cl 2 O 7 is a highly reactive compound used in some organic synthesis reactions. Calculate the percent composition of dichlorineheptoxide. Percent composition can also be used to determine the mass of a certain element that is contained in any mass of a compound. In the previous sample problem, it was found that the percent composition of dichlorineheptoxide is Suppose that you needed to know the masses of chlorine and oxygen present in a You can set up a conversion factor based on the percent by mass of each element.
The sum of the two masses is Use the link below to review material and do calculations. Read both parts of the lesson and do as many calculations as you have time for. Why does the color change? If you look at a typical bottle of copper sulfate, it will be a bluish-green. You are both right; it just depends on the copper sulfate. Why the difference? The water molecules interact with some of the d electrons in the copper ion and produce the color.
When the water is removed, the electron configuration changes and the color disappears. Many ionic compounds naturally contain water as part of the crystal lattice structure. A hydrate is a compound that has one or more water molecules bound to each formula unit. Ionic compounds that contain a transition metal are often highly colored.
Interestingly, it is common for the hydrated form of a compound to be of a different color than the anhydrous form, which has no water in its structure. A hydrate can usually be converted to the anhydrous compound by heating. For example, the anhydrous compound cobalt II chloride is blue, while the hydrate is a distinctive magenta color.
On the left is anhydrous cobalt II chloride, CoCl 2. The hydrated form of cobalt II chloride contains six water molecules in each formula unit. The formula for water is set apart at the end of the formula with a dot, followed by a coefficient that represents the number of water molecules per formula unit.
It is useful to know the percent of water contained within a hydrate. The sample problem below demonstrates the procedure. The mass of water in the hydrate is the coefficient 6 multiplied by the molar mass of H 2 O. The molar mass of the hydrate is the molar mass of the CoCl 2 plus the mass of water. Nearly half of the mass of the hydrate is composed of water molecules within the crystal. Use the following link to practice calculating percent water in a hydrate:.
What is occuring in this picture? In the early days of chemistry, there were few tools for the detailed study of compounds. Much of the information regarding the composition of compounds came from the elemental analysis of inorganic materials. The relative amounts of elements could be determined, but so many of these materials had carbon, hydrogen, oxygen, and possibly nitrogen in simple ratios.
We did not know exactly how many of these atoms were actually in a specific molecule. An empirical formula is one that shows the lowest whole-number ratio of the elements in a compound. Because the structure of ionic compounds is an extended three-dimensional network of positive and negative ions, all formulas of ionic compounds are empirical.
However, we can also consider the empirical formula of a molecular compound. Ethene is a small hydrocarbon compound with the formula C 2 H 4 see Figure below. While C 2 H 4 is its molecular formula and represents its true molecular structure, it has an empirical formula of CH 2.
The simplest ratio of carbon to hydrogen in ethene is There are two ways to view that ratio. Considering one molecule of ethene, the ratio is 1 carbon atom for every 2 atoms of hydrogen. Considering one mole of ethene, the ratio is 1 mole of carbon for every 2 moles of hydrogen.
So the subscripts in a formula represent the mole ratio of the elements in that formula. Ball-and-stick model of ethene, C 2 H 4. In a procedure called elemental analysis , an unknown compound can be analyzed in the laboratory in order to determine the percentages of each element contained within it. These percentages can be transformed into the mole ratio of the elements, which leads to the empirical formula.
The steps to be taken are outlined below. A compound of iron and oxygen is analyzed and found to contain Find the empirical formula of the compound. Steps to follow are outlined in the text. Divide both moles by the smallest of the results.
Since the moles of O, is still not a whole number, both moles can be multiplied by 2, while rounding to a whole number. The empirical formula of the compound is Fe 2 O 3. The subscripts are whole numbers and represent the mole ratio of the elements in the compound. The compound is the ionic compound iron III oxide. Use the link below to read about calculating empirical formulas and practice working some problems:. How can you determine the differences between these two molecules?
Above we see two carbohydrates: glucose and sucrose. Sucrose is almost exactly twice the size of glucose, although their empirical formulas are very similar. The best way is to determine the molecular weights — this approach allows you to easily tell which compound is which. Molecular formulas give the kind and number of atoms of each element present in a molecular compound.
In many cases, the molecular formula is the same as the empirical formula. The molecular formula of methane is CH 4 and because it contains only one carbon atom, that is also its empirical formula. Sometimes, however, the molecular formula is a simple whole-number multiple of the empirical formula.
Acetic acid is an organic acid that is the main component of vinegar. Its molecular formula is C 2 H 4 O 2. Glucose is a simple sugar that cells use as a primary source of energy. Its molecular formula is C 6 H 12 O 6.
The structures of both molecules are shown in the figure below. They are very different compounds, yet both have the same empirical formula of CH 2 O. Acetic acid left has a molecular formula of C 2 H 4 O 2 , while glucose right has a molecular formula of C 6 H 12 O 6. Both have the empirical formula CH 2 O. Empirical formulas can be determined from the percent composition of a compound. In order to determine its molecular formula, it is necessary to know the molar mass of the compound.
Chemists use an instrument called a mass spectrometer to determine the molar mass of compounds. In order to go from the empirical formula to the molecular formula, follow these steps:. The empirical formula of a compound of boron and hydrogen is BH 3.
Its molar mass is Determine the molecular formula of the compound.
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