# 6: Chemical Composition


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Chemical composition refers to the arrangement, type, and ratio of atoms in molecules of chemical substances. Chemical composition varies when chemicals are added or subtracted from a substance, when the ratio of substances changes, or when other chemical changes occur in chemicals.

• 6.1: How Much Sodium?
Why is Knowledge of Composition Important? everything in nature is either chemically or physically combined with other substances to know the amount of a material in a sample, you need to know what fraction of the sample it is Some Applications: the amount of sodium in sodium chloride for diet the amount of iron in iron ore for steel production the amount of hydrogen in water for hydrogen fuel the amount of chlorine in freon to estimate ozone depletion.
• 6.2: Counting Nails by the Pound
The size of molecule is so small that it is physically difficult if not impossible to directly count out molecules. However, we can count them indirectly by using a common trick of "counting by weighing".
• 6.3: Counting Atoms by the Gram
In chemistry, it is impossible to deal with a single atom or molecule because we can't see them or count them or weigh them. Chemists have selected a number of particles with which to work that is convenient. Since molecules are extremely small, you may suspect this number is going to be very large and you are right. The number of particles in this group is Avagadro's number and the name of this group is the mole.
• 6.4: Counting Molecules by the Gram
The molecular mass of a substance is the sum of the average masses of the atoms in one molecule of a substance. Calculations for formula mass and molecular mass are described. Calculations involving conversions between moles of a material and the mass of that material are described. Calculations are illustrated for conversions between mass and number of particles.
• 6.5: Chemical Formulas as Conversion Factors
Using formulas to indicate how many atoms of each element we have in a substance, we can relate the number of moles of molecules to the number of moles of atoms. In any given formula the ratio of the number of moles of molecules (or formula units) to the number of moles of atoms can be used as a conversion factor.
• 6.6: Mass Percent Composition of Compounds
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.
• 6.7: Mass Percent Composition from a Chemical Formula
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. That is divided by the molar mass of the compound and multiplied by 100%.
• 6.8: Calculating Empirical Formulas for Compounds
An empirical formula tells us the relative ratios of different atoms in a compound. The ratios hold true on the molar level as well. A process is described for the calculation of the empirical formula for a compound based on the percent composition of that compound.
• 6.9: Calculating Molecular Formulas for Compounds
A procedure is described that allows the calculation of the exact molecular formula for a compound.

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