Complete the following practice problems. Remember the mole bridge, the conversions factors, and the steps for dimensional analysis. Use your Chemistry STAAR reference material to help you.

Dimensionsal Analysis Steps

1. Write the given information as a fraction by placing it over 1. (Placing it over 1 makes it a fraction but does not change its value.)
2. Write a conversion factor that has the unit you want to remove in the denominator and the unit you want to end up with in the numerator. After you fill in your units, add the numbers. (Usually one of the numbers is a 1, but it can be in either the denominator or the numerator.) Note: In some cases you may need to repeat this step a number of times in order to get the unit you want to end up with in the numerator.
3. Mark through the units to double check that they all cancel and that you are left with the units you want.
4. Multiply the numbers in the numerators, and then multiply the numbers in the denominators.

1. How many atoms are in a pure gold necklace that has a mass of 25.0 grams?

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grams → moles → particles (atoms)

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25.0 g Au over 1 25.0 g Au 1 ×1 mole Au over 197.0 g Au 1 mole Au 197.0 g Au ×6.02 × 1023 atoms Au over 1 mole Au 6.02 × 1023 atoms Au 1 mole Au = 7.64 × 1022 Au atoms

2. How many grams of lithium are in 7.56 x 1023 atoms of lithium?

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particles (atoms) → moles → grams

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7.56 × 1023 atoms Li over 1 7.56 × 1023 atoms Li 1 × 1 mole Li over 6.02 × 1023 atoms Li 1 mole Li 6.02 × 1023 atoms Li ×7.0 g Li over 1 mole Li 70.0 g Li 1 mole Li = 8.79 g Li

3. Find the mass of 7.80 x 1045 molecules of water.

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particles (molecules) → moles → grams

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7.80 × 1045 Molecules H2O over 1 7.80 × 1045 Molecules H2O 1 × 1 mole H2O over 6.02 × 40.1 g Ca 1 mole H2O 6.02 × 1023 Molecules H2O ×18.0 g H2O over 1 mole H2O 18.0 g H2O 1 mole H2O = 2.33 × 1023 g H2O

4. If a sample of calcium has a mass of 13.3 grams, how many atoms of calcium are present?

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grams → moles → particles (atoms)

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13.3 g Ca over 1 13.3 g Ca 1 × 1 mole Ca over 6.02 × 40.1 g Ca 1 mole Ca 40.1 g Ca ×6.02 × 1023 atoms Ca over 1 mole Ca 6.02 × 1023 atoms Ca 1 mole Ca = 1.99 × 1023 Ca atoms

5. Calculate the volume (at STP) in liters that 2.3 g of NO2 would occupy.

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grams → moles → liters

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2.3 g NO2 over 1 2.3 g NO2 1 × 1 mole NO2 over 46.0 g NO2 1 mole NO2 46.0 g NO2 ×22.4 L NO2 over 1 mole NO2 22.4 L NO2 1 mole NO2 = 1.1 L NO2

6. How much would 46.8 L (at STP) of N2O5 weigh in grams?

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liters → moles → grams

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13.3 g Ca over 1 46.8 L N2O5 1 × 1 mole Ca over 6.02 × 40.1 g Ca 1 mole N2O5 22.4 L N2O5 ×6.02 × 1023 atoms Ca over 1 mole Ca 108.0 g N2O5 1 mole N2O5 = 226.0 g N2O5

7. If a steel gas tank can hold 52.0 L (at STP) of hydrogen gas, how many molecules of H2 are in the canister?

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liters → moles → particles (molecules)

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52.0 L H2 over 1 52.0 L H2 1 × 1 mole H2 over 22.4 L H2 1 mole H2 22.4 L H2 ×6.02 × 1023 Molecules H2 over 1 mole H2 6.02 × 1023 Molecules H2 1 mole H2 = 1.4 × 1024 molecules H2

Use the information given and your periodic table to complete the table. Be sure to use the correct number of significant figures.