The density of a material provides information on how closely packed the material's molecular building blocks are. A mole of any material always contains the same number of molecular building blocks, but will differ in mass depending on the density of the material. Molar quantities, which provide a way for chemistry to downsize the large numbers associated with quantifying the atoms or molecules that comprise the building blocks of materials, may be related to the density of a material through measurable quantities such as mass and volume.
Things You'll Need
- Periodic table
- Graduated beaker
Measure the volume of the sample. If the sample is a powder or a liquid, the volume is measured by pouring the sample into a clean, dry graduated beaker and reading the volume directly from the markings on the beaker. If the sample is an irregular solid like a rock or metal object, the volume can be measured by submerging the specimen in a beaker of water and measuring the volume displacement. The unit of the volume should be given in cubic centimeters.
Multiply the volume (in cubic centimeters) by the density (grams per cubic centimeter) of the substance to obtain the mass (in grams) of the sample. For example, 138.6 cubic cm of table salt x 2.165 g/(cm^3) = 300 grams of salt.
Calculate the molecular weight or molar mass of the specimen. To do this, first determine how many atoms of each different element are in the chemical formula of the substance. Then find the atomic weight of each element by consulting the periodic table. The atomic weight is the larger of the two numbers associated with the chemical symbol of each element in the periodic table. Multiply the number of atoms of each element in the formula by the atomic weight of each element and add the products to obtain the molecular weight. For example, table salt (NaCl) has one atom of sodium (Na) and one atom of chlorine (Cl) in the formula. The molecular weight for NaCl = 1 x (22.9 g/mol) + 1 x (35.5 g/mol) = 58.4 g/mol. The molecular weight is the mass of one mole of the substance.
Divide the mass of the sample (given in grams) by the molecular weight of the sample (in grams per mole or g/mol). The quotient is the number of moles of substance in the sample. For example, 300 grams of NaCl is equivalent to (300 g)/(58.4 g/mol) = 5.1 moles of table salt.
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