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Sample Problem 2.2 Converting from Celsius to Fahrenheit Temperature
Solution
During the winter, the thermostat in a room is set at 21 °C. To what temperature, in Fahrenheit degrees, should you set the thermostat?
In the process of making ice cream, rock salt is added to crushed ice to chill the ice cream mixture. If the temperature drops to what is the temperature in degrees Fahrenheit?
Study Check 2.2
Step 1 State the given and needed quantities.Analyze the Problem
Step 2 Write a temperature equation.
Step 3 Substitute in the known values and calculate the new temperature. In the equation, the values of 1.8 and 32 are exact numbers , which do not affect the number of SFs.
Sample Problem 2.3 Converting from Fahrenheit to Celsius Temperature
Solution
In a type of cancer treatment called thermotherapy , temperatures as high as 113 °F are used to destroy cancer cells. What is that temperature in degrees Celsius?
A child has a temperature of 103.6 °F. What is this temperature on a Celsius thermometer?
Study Check 2.3
Step 1 State the given and needed quantities.Analyze the Problem
Step 2 Write a temperature equation.
Step 3 Substitute in the known values and calculate the new temperature.
Sample Problem 2.4 Converting from Celsius to Kelvin Temperature
Solution
A dermatologist may use cryogenic liquid nitrogen at to remove skin lesions and some skin cancers. What is the temperature, in kelvins, of the liquid nitrogen?
Study Check 2.4
Step 1 State the given and needed quantities.Analyze the Problem
Step 2 Write a temperature equation.
Step 3 Substitute in the known values and calculate the new temperature.
On the planet Mercury, the average night temperature is 13 K, and the average day temperature is 683 K. What are these temperatures in degrees Celsius?
a. Copper is an element, which is a pure substance.b. A chocolate chip cookie does not have a uniform composition, which makes it a heterogeneousmixture.c. The gases oxygen and nitrogen have a uniform composition in Nitrox, which makes it a homogeneousmixture.
Solution
Classify each of the following as a pure substance (element or compound) or as a mixture (homogeneous or heterogeneous):
A salad dressing is prepared with oil, vinegar, and chunks of blue cheese. Is this a homogeneous orheterogeneous mixture?
Study Check 2.5
a. copper wireb. a chocolate chip cookiec. Nitrox, a breathing mixture of oxygen and nitrogen for scuba
a. A physical change occurs when the ice cube changes state from solid to liquid.b. A chemical change occurs when an enzyme breaks down lactose into simpler substances.c. A physical change occurs when the size of an object changes.
Solution
Classify each of the following as a physical or chemical change:
a. An ice cube melts to form liquid water.b. An enzyme breaks down the lactose in milk.c. Peppercorns are ground into flakes.
Study Check 2.6
Which of the following are chemical changes?
a. Gas bubbles form when baking powder is mixed with vinegar.b. A log is chopped for firewood.c. A log is burned in a fireplace.
What is the specific heat, in cal/g °C, of lead if 13.6 cal will raise the temperature of35.6 g of lead by 12.5 °C?
Step 1 State the given and needed quantities.Analyze the Problem
Step 2 Write the relationship for specific heat. In the relationship for specific heat ( SH ), the quantity of heat is divided by the mass and by the temperature change
What is the specific heat of sodium metal (J/g °C) if 123 J are needed to raise the temperatureof 4.00 g of sodium by 25.0 °C?
Study Check 2.7
Step 3 Set up the problem to calculate the specific heat. Substitute the quantity of heat, in calories, the mass, in grams, and the temperature change in degrees Celsius, into the relationship for specific heat.
Sample Problem 2.8 Calculating Heat with an Increase in Temperature
Continued
Some cooking pans have a layer of copper on the bottom. How many kilojoules are needed to raise the temperature of 125 g of copper from 22 °C to 325 °C (see Table 2.8)?
Study Check 2.8
Step 4 Substitute in the given values and solve, making sure units cancel.
At a fast-food restaurant, a hamburger contains 37 g of carbohydrate, 19 g of fat, and 24 g of protein. What is the energy content for each food type and the total energy content, in kcal? Round off the kilocalories for each food type to the tens place.
Step 1 State the given and needed quantities.Analyze the Problem
Using the energy values for carbohydrate, fat, and protein (see Table 2.9), we can calculate the energy for each type of food.
Step 2 Use the energy value of each food type and calculate the kJ or kcal rounded off to the tens place.
Step 3 Add the energy for each food type to give the total energy for the food.
Total energy content = 150 kcal + 170 kcal + 100 kcal
= 420 kcal
Study Check 2.9
If you buy the same hamburger as described in Sample Problem 2.9 at a fast-food restaurant in Canada, what is the energy content for each food type and the total energy content, in kJ? Round off the kilojoules for each food type to the tens place.