Exam # 1 CH 100, Introductory Chemistry, Fall, 2003 Name Be sure to show your set up for all mathematical problems. Your answers must have the correct number of significant digits and the correct units. I. Chemistry is a quantitative science therefore we must make measurements. All measurements have an uncertainty that we need to be aware of in our calculations. A. We need to be able to determine the number of significant figures in a measurement. For the following measurement state the number of significant figures. (4pts) 1. 1.000 cm 4 2. 2.590 x 10-5 m 4 3. 0.020 g 2 4. 1000 cm 1 B. We need to be able to add, subtract, multiply, and divide with measurements. Solve the following problems. (4 pts) 1. 3.02 cm + 5 cm + 0.002 cm = 8 cm 3. 2.0 cm x 2.00 cm x 2.000 cm = 8.0 cm 3 2. 5.35 g - 0.005 g = 5.35 g 4. 7.89 g / 5.2 ml = 1.5 g/ml B. It is often necessary to convert from one unit to another. This is easily accomplished with the right unit factor. Solve the following problems. 1. During the last six lunar landings 842 pounds of Moon samples have been collected. What is this mass expressed in kilograms? ( 2.2 pounds = 1 kilograms) (4 pts) 842 lbs x 1 kg / 2.2 lbs = 382.7272727 kg = 380 kg 2. If a 250 ml beaker weighs 95.4 g, what is the mass in kilograms? (1 kg = 1000 g) (4 pts) 95.4 g x 1kg / 1000g = 0.0954 kg = 9.54 x 10-3 kg 3. If the radius of an oxygen atom is 6.6 x 10-10 dm, what is the radius in nm? (1 m = 10 dm, 1 m = 10 9 nm) (5 pts) 6.6 x 10 10 dm x 1 m / 10 dm x 10 9 nm/ 1 m = 6.6 x 10-2 nm 1
C. Chemists often deal with percent problems. As long as you remember that percent refers to per 100, these problems can be solved. 1. A penny minted after 1982 is composed of copper and zinc only. If a penny has a mass of 2.507 g and is 2.50 % copper. What is the mass of zinc in the coin? (5 pts) 2.507 g x 2.50/100 = 0.062675 g of Cu 2.507 g of penny - 0.062675 g of Cu = 2.444325 g = 2.444 g of Zn (2.44 g of Zn is correct also) D. Density is a very important concept for chemist. The density of a substance can be obtained in a chemistry laboratory. 1. A rubber stopper with a mass of 23.75 g is dropped in a 50-mL graduated cylinder that has 20.4 ml of water. After the stopper is dropped in the graduated cylinder, the water level rises to 24.7 ml. What is the density of the rubber stopper? (5 pts) 2.375 g / (24.7 ml 20.4 ml) = 5.523255814 g / ml = 5.52 g / ml (5.5 g /ml is accepted) We can make predictions if the density is known. 2. A glass cylinder contains four liquid layers: mercury (d = 13.6 g/ml), chloroform (d = 1.49 g/ml), water (d = 1.00 g/ml), ether (d = 0.708 g/ml). If a piece of ice (d = 0.917 g/ml) is dropped into the cylinder, where does it come to rest? Why? (5 pts) The mercury layer will be on the bottom, then the chloroform, then the water, and finally the ether. The ice will sink in the ether layer and float on top of the water layer. If the density is know, we can we can find the mass of a given volume or the volume of a given mass. 3. The density of aluminum is 2.70 g/ml. What volume will 250 g of Al occupy? What is the mass of a piece of aluminum with a volume of 250 ml? (6 pts) 250 g of Al x 1 ml / 2.70 g = 92.59259259 ml = 93 ml 250 ml of Al x 2.70 g / 1 ml = 675 g = 680 g 2
Using the density value we can indirectly obtain distances that are too small to measure directly. 4. A sample of aluminum foil has a length of 10.0 cm and a width of 5.7 cm. If the aluminum foil weighs 0.234 g, what is the thickness of the aluminum foil? Remember that the density of aluminum is 2.70 g/ml. (6 pts) Volume of Al = length x width x thickness = mass / density 0.234 g x 1 ml / 2.70 g = 0.0866666666 ml 0.086666666 ml / 10.0 cm x 5.7 cm = 0.001520467 cm = 1.5 x 10 3 cm E. Heat and temperature are two very important concepts. 1. In your own words, please state the difference between heat and temperature. What is a common unit of each? (5 pts) Heat measures the total energy of a sample and temperature measure the intensity of heat or the average heat of a sample. A common unit of heat is a calorie and a common unit of temperature is Kelvin. It is important to be able to convert from one temperature unit to another. 2. Aluminum melts at 1220 o F. (You can see why aluminum is not a liquid at room temperature) What is the melting point of aluminum in degree Celsius ( o C) and in Kelvin (K)? ( o F 32 o F) x 100 o C = o C K = o C + 273 (6 pts) 180 o F (1220 o F 32)(100 / 180) = 660 o C 660 o C + 273 = 933 K We can also calculate the amount of heat required to raise the temperature of a substance if we know the specific heat of the substance. 3. Define the term specific heat. (5 pts) Specific heat is the amount of heat required to bring about a given change in temperature. We can also define the specific heat as the amount of heat required to raise the temperature of one gram of substance one degree Celsius. 3
4. The specific heat of water is 1.00 cal/g o C. How many calories of heat are required to raise the temperature of 155 g of water to make a cup of tea from room temperature (20.5 o C) to hot (95.5 o C)? (5 pts) 1.00 cal / o C x 155 g x (95.5 20.5) o C = 11625 cal = 1.16 x 10 4 cal 5. In performing a multistep calculation, when should you round off the answer in the calculator display? (5 pts) In a multistep calculation you should round off the answer at the end of the calculation E. Matter and Energy. 1. We learned four terms in chapter 4, heterogeneous mixture, homogeneous mixture, compound, and element. Please an example of each. (8 pts) An example of a heterogeneous mixture is the earth s crust. A heterogeneous mixture can be separated into pure substances by physical methods. An example of a homogeneous mixture is salt water. The properties of a homogeneous mixture constant for a given sample. An example of a compound is NaCl. A compound can be broken down into elements by ordinary chemical reactions. An example of an element is Na. An element is a substance that can not be broken down further by a chemical reaction. 2. Match Symbols and Names (10 pts) 1. sodium d 2. iron c 3. carbon e 4. nitrogen b 5. phosphorous a a. P b. N c. Fe d. Na e. C 4
1. Cl e 2. Ce c 3. Cu b 4. Co d 5. Ca a a. Calcium b. Copper c. Cesium d. Cobalt e. Chlorine 3. Give an example for each of the following terms: (8 pts) a. physical property Appearance, melting point, boiling point, density, heat and electrical conductivity, solubility, and physical state under normal conditions are all examples of physical properties. b. chemical property A chemical property of a substance describes its chemical reactions with other substances. The chemical property of sodium states that sodium will react with water to form hydrogen gas. c. physical change A physical change include changing shape, volume, or physical state. For example when water changes to ice. d. chemical change - A chemical change always involves the formation of a new substance. Gas bubbles from the reaction of calcium with water. 5