# 6: Chemical Composition

- Page ID
- 323293

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Chapter 6 is concerned with the amounts of substances which participate in chemical reactions, the quantities of heat given off or absorbed when reactions occur, and the volumes of solutions which react exactly with one another. These seemingly unrelated subjects are discussed together because many of the calculations involving them are almost identical in form. The same is true of the density calculations, and of the calculations involving molar mass and the Avogadro constant.

- 6.1: Prelude to Chemical Composition - How Much Sodium?
- Why is knowledge of composition important? Everything in nature is either chemically or physically combined with other substances. To find the amount of a material in a sample, you need to know what fraction of the sample it is.

- 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: 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.

- 6.6: 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. This is divided by the molar mass of the compound and multiplied by 100%.

- 6.7: 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.

- 6.8: Calculating Molecular Formulas for Compounds
- A procedure is described that allows the calculation of the exact molecular formula for a compound.