|Science Focus Topic 6 Notes: Density||| Print ||
When we talk about density, it's usually mass density we're referring to. The mass density of an object is simply
it's mass divided by it's volume.
Density is the mass of a substance divided by its volume. This changes as temperature changes, therefore, any stated density should be accompanied by the appropriate units in which it was determined, together with the temperature at which it was determined.
Key Concepts to be covered:
Measurement of Density
There are physical characteristics of a substance that help identify the substance. One of these characteristics is density. Density (whose symbol is the lowercase letter d) is defined as mass per unit volume. Density is calculated by dividing the mass of an object by its volume. This is shown in equation form, as follows:
Density = mass/volume
This simple equation will help you figure out how to solve density problems:
For example: cover up the M. This leaves you with d/V (ignore the fact that it is in the de-nominator).
Density times volume will give you mass. You can also check it out by way of the units: (g / cm3)
x cm3 cancels out the volume unit leaving grams, the desired unit for mass.
Practice Calculation Problems:
1) A block of aluminum occupies a volume of 15.0 mL and weighs 40.5 g. What is its density?
2) Mercury metal is poured into a graduated cylinder that holds exactly 22.5 mL. The mercury used to fill the cylinder weighs 306.0 g. From this information, calculate the density of mercury.
3) What is the weight of the ethyl alcohol that exactly fills a 200.0 mL container? The density of ethyl alcohol is 0.789 g/mL.
4) A rectangular block of copper metal weighs 1896 g. The dimensions of the block are 8.4 cm by 5.5 an by 4.6 cm. From this data, what is the density of copper?
5) A flask that weighs 345.8 g is filled with 225 mL of carbon tetrachloride. The weight of the flask and carbon tetrachloride is found to be 703.55 g. From this information, calculate the density of carbon tetrachloride.
6) Calculate the density of sulfuric acid if 35.4 mL of the acid weighs 65.14 g.
7) Find the mass of 250.0 mL of benzene. The density of benzene is 0.8765 g/mL.
8) A block of lead has dimensions of 4.50 cm by 5.20 cm by 6.00 cm. The block weighs 1587 g. Front this inforination, calculate the density of lead.
9) 28.5 g of iron shot is added to a graduated cylinder containing 45.50 mL of water. The water level rises to the 49. 10 mL mark, From this information, calculate the density of iron.
10) What volume of silver metal will weigh exactly 2500.0 g. The density of silver is 10.5 g/CM3.
Answers to the Practice Density Problems
A hydrometer is a device that uses buoyancy to measure density directly p. 82 ... Activity 2-1 and 2-2
Hydrometers are calibrated in g/ml, by making marks that indicate the levels at which the instrument floats in different fluids. p.83
The higher the hydrometer floats, the higher the density of the liquid.
A liquid can exert a buoyant force making objects (solids, liquids or gases) float, p. 14 Activities 2-6/2-7 /2-8
Density depends, in part, on the concentration of the solution (how much solute is dissolved in a certain solvent) the more concentrated the solution. the more dense it is, Pages 789 819 82,, 85
Ships can float because they contain large volume of air. The overall density of the ship is less dense than water, so it floats.
The buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object. p. 82
Using a hydrometer to measure density directly is important in industrics such as plastics.
Hot-Air Balloons p. 83