Introduction to Atomic Mass

Description

Students will find the "atomic mass" of three different types of objects -- peas, beans, and rice. Each of these objects is too small to mass accurately one by one, so a larger sample is massed and divided by the number of objects in the sample to find the individual mass. The calculated "atomic mass" is then used to estimate the number of objects in a sample of known mass.

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Pick up any spilled materials.

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1. Weigh an empty paper cup.
2. Find the mass of 20 dried beans.
3. Set your balance at a mass determined by the formula, mass-of-one-bean x 100.
4. Add enough beans to the pan to balance the beam.
5. Count the number of beans on the balance.
6. Repeat these same steps using dried peas.
7. Repeat again using dried rice.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 016 Introduction to Atomic Mass

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Closure Questions:

1. Twelve peas have a mass of 48 g. Find the mass of 6 peas.
2. Find the mass of 100 grains of rice if 250 grains have a mass of 7.5 g.
3. A mass of 60 g of beans contains how many beans, if each one has a mass of 0.3 g?
4. The mass of 1 grain of rice is 0.020 g. How many grains are in 1 lb (0.453 kg)?
5. The mass of 6.02 x 10²³ atoms of copper is 63.5 g. Find the mass of one atom.
6. The percentage error in the predicted number of "seeds" for this experiment is equal to the absolute value of the following expression:

   (Counted number - 100)%

   1. Determine the percentage error for each sample in your chart.
   2. Which was the least accurate?
   3. Suggest factors that might result in this being the least accurate.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 016 Introduction to Atomic Mass

Watch the movie, and answer the questions.

1. How can we find the mass of a very small object if our balance is not sufficiently sensitive to determine the mass of one of the objects?
2. Find the mass of 1 bean if 20 beans have a mass of 44g.

Use this sample data to answer the questions below.

1. Twelve peas have a mass of 48 g. Find the mass of 6 peas.
2. Find the mass of 100 grains of rice if 250 grains have a mass of 7.5 g.
3. A mass of 60 g of beans contains how many beans, if each one has a mass of 0.3 g?
4. The mass of 1 grain of rice is 0.020 g. How many grains are in 1 lb (0.453 kg)?
5. The mass of 6.02 x 10²³ atoms of copper is 63.5 g. Find the mass of one atom.
6. The percentage error in the predicted number of "seeds" for this experiment is equal to the absolute value of the following expression:

   (Counted number - 100)%

   1. Determine the percentage error for each sample in your chart.
   2. Which was the least accurate?
   3. Suggest factors that might result in this being the least accurate.

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Purpose

• To determine equivalent of the "atomic mass" of a dried pea, a dried bean, and a grain of rice.
• To use these values to estimate the number of peas or beans or grains in a sample.

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(per 10 students working in pairs)

• 5 balance
• peas (about 600)
• beans (about 600)
• rice (about 600 grains)
• (beads of three different sizes and/or colors from craft stores may be used instead)
• 15 paper cup

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• When students are using rice, be sure they use whole grains only -- not broken pieces. You may estimate the amount you will need by counting 50, massing them, multiplying by 12, and the measuring out that mass.
• Some plastic beads and some dry cereals are also suitable for use.

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Teacher preparation: 15 minutes

Class Time: 30-40 minutes

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There are no unusual hazards in this experiment.

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No special precautions are required in this experiment. Follow routine laboratory precautions. Caution students to pick up any spilled materials.

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Save peas, beans and rice in covered containers for use in future classes.

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Set Questions:

1. How can we find the mass of a very small object if our balance is not sufficiently sensitive to determine the mass of one of the objects?
2. Find the mass of 1 bean if 20 beans have a mass of 44g.

Answers to Set Questions:

1. Weigh a specific number of the objects that can be weighed accurately; then divide by the number in the sample.
2. 44 g/20 beans = 2.2 g/bean

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A good estimate of the mass of a very small object can be obtained by weighing a large number of them and calculating the mass of a single one.

Answers to Closure Questions: (based on sample data above)

1. 48 g/12 peas = 4.0 g/pea

   4.0 g/pea x 6 peas = 24 g

2. 7.5 g/250 grains = 0.030 g/grain

   0.030 g/grain x 100 grains = 3.0 grams

3. (60 grams) x (1 bean/0.3 g) = 200 beans
4. (1.00 lb) (453 g/1 lb) (1 grain/0.020 g) = 2.3 x 10⁴
5. 63.5 g/6.02 x 10²³ atoms = 1.05 x 10^-22g/atom
6. 1. 01-100 = 1% error for peas; 99-100 = 1% error for beans; 111-100 = 11% error for rice.
   2. Rice was the least accurate.
   3. The grains of rice may have been less uniform in mass. They are more likely to be broken during milling to remove the outside part. Also, the relation of systematic balance error to measured mass is much larger for the 1.75 g mass than for the 47 g mass.

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Closure Questions:

1. Twelve peas have a mass of 48 g. Find the mass of 6 peas.
2. Find the mass of 100 grains of rice if 250 grains have a mass of 7.5 g.
3. A mass of 60 g of beans contains how many beans, if each one has a mass of 0.3 g?
4. The mass of 1 grain of rice is 0.020 g. How many grains are in 1 lb (0.453 kg)?
5. The mass of 6.02 x 10²³ atoms of copper is 63.5 g. Find the mass of one atom.
6. The percentage error in the predicted number of "seeds" for this experiment is equal to the absolute value of the following expression:

   (Counted number - 100)%

   1. Determine the percentage error for each sample in your chart.
   2. Which was the least accurate?
   3. Suggest factors that might result in this being the least accurate.

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• mass
• atomic mass
• mole

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