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Review of Density

Review of Density. Dr. C. Yau Spring 2015 (Roughly based on Chap.1 Sec. 7 from Jespersen 7 th edition). Density. Density is defined as mass per unit volume . This is expressed as the equation:. where M = mass of the sample V = volume of the sample.

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Review of Density

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  1. Review of Density Dr. C. Yau Spring 2015 (Roughly based on Chap.1 Sec. 7 from Jespersen 7th edition)

  2. Density Density is defined as mass per unit volume. This is expressed as the equation: where M = mass of the sample V = volume of the sample D is generally in g/L for gases. g/mL for liquids g/cm3 for solids (or g cm-3) Generally g/mL or g/cm3 are used interchangeably for liquids & solids. Why are the units different for the 3 physical states?

  3. Density: Table 1.6 Do you see a pattern in the densities? What type of substance has a very low density? What type of substance has a high density? gas Natural Gas 0.0066 Alcohol 0.789 Gasoline 0.737 Air 0.0012 solid = 1.2 g/L

  4. Why are densities so different for the different physical states? solid liquid gas Solids have atoms packed closely together. Gases have atoms very far apart, Liquids are in between. For the same volume, solids have many more atoms (with mass) than gases. It is much more dense. Gases contain mostly empty spaces.

  5. Why specify room temp? Density What has temp to do with density? It is affected by temperature.

  6. Density Varies with Temperature TABLE 1.7: Density of Water as a Function of Temperature (p.51) Why does density vary with temperature?

  7. Density Varies with Temperature Let us consider the two terms in the equation: What effect does temperature have on M? on V? When it gets hotter, the volume expands. When T increases, V increases also, but not M. Thus, the denominator increases, but not M. The result is the fraction M/V would get smaller.

  8. Density Varies with Temperature The previous explanation is based on how numerically M and V affect D. How do we explain this effect conceptually? As T increases, particles (atoms/molecules) move faster. The added energy causes them to weaken the forces holding the particles together. At the bp of the substance, the energy added is sufficient to break the forces completely, and the particles move very far from each other and D becomes v. low.

  9. Density Density is an intensive property. An intensive property is one that does not depend on the size of the sample. Example: The density of one drop of water is the same as the density of one gallon of water.

  10. Uncertainty in Measurement What is opposite to "intensive property?" An extensive property is one that depends on the size of the sample. Think of a property that does depend on size. mass volume

  11. Density Calculations You have already seen how density can be used as a conversion factor for converting mass to volume and vice versa. Let us look at one further density problem using dimensional analysis.

  12. A glass bead with a mass of 5.96 g is dropped into a beaker of water containing 10.2 mL. If the resulting volume is 12.3 mL, what is the density of the bead? What is the unit we are looking for?What unit should the answer have? What is wrong with this answer? What checks do we need to make? Ans. 2.8 g/mL

  13. Density Calculations 7. Acetone, the solvent in some nail polish removers, has a density of 0.790 g/mL. What is the volume of 15.8 g of acetone? Using dimensional analysis, how would you start on this problem? Now find the volume, using the equation, D = M/V. Ans. not 20 mL What is wrong with starting this way? Ans. is 20.0 mL

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