## Mole Calculations Worksheet Answer Key

If you are studying chemistry, you have likely come across mole calculations. Mole calculations involve determining the amount of a substance in moles based on various factors such as molar mass, volume, and concentration. This worksheet answer key provides solutions to common mole calculation problems to help you understand the concept better.

### Problem 1:

Calculate the number of moles in 25.0 grams of water (H2O).

**Solution:**
To calculate the number of moles, we first need to determine the molar mass of water. The molar mass of water is 18.015 g/mol. Next, we use the formula:

Number of moles = Mass / Molar mass

Plugging in the values, we get:

Number of moles = 25.0 g / 18.015 g/mol = 1.387 moles

Therefore, there are 1.387 moles of water in 25.0 grams.

### Problem 2:

Calculate the volume of 3.00 moles of hydrogen gas (H2) at STP.

**Solution:**
At STP (standard temperature and pressure), one mole of any gas occupies 22.4 liters. Using this information, we can calculate the volume of 3.00 moles of hydrogen gas:

Volume = Number of moles x 22.4 liters/mole

Plugging in the values, we get:

Volume = 3.00 moles x 22.4 liters/mole = 67.2 liters

Therefore, 3.00 moles of hydrogen gas at STP occupy 67.2 liters.

### Problem 3:

What is the molarity of a solution made by dissolving 30.0 grams of sodium chloride (NaCl) in enough water to make 500.0 mL of solution?

**Solution:**
To calculate the molarity of the solution, we need to determine the number of moles of sodium chloride first. The molar mass of NaCl is 58.44 g/mol. Using the formula:

Number of moles = Mass / Molar mass

We find:

Number of moles = 30.0 g / 58.44 g/mol = 0.513 moles

Next, we need to convert 500.0 mL to liters (1 L = 1000 mL):

Volume = 500.0 mL / 1000 mL/L = 0.500 L

Finally, we calculate the molarity using the formula:

Molarity = Number of moles / Volume of solution

Plugging in the values, we get:

Molarity = 0.513 moles / 0.500 L = 1.026 M

Therefore, the molarity of the solution is 1.026 M.

### Problem 4:

A sample of iron (Fe) contains 2.50 x 10^23 atoms. What is the mass of the sample in grams?

**Solution:**
To determine the mass of the sample, we first need to convert the number of atoms to moles. One mole of any element contains Avogadro’s number of atoms, which is 6.022 x 10^23. Using this conversion factor, we find:

Number of moles = Number of atoms / Avogadro’s number

Number of moles = 2.50 x 10^23 atoms / 6.022 x 10^23 atoms/mol = 0.415 moles

Next, we need to calculate the molar mass of iron, which is 55.85 g/mol. Using the formula:

Mass = Number of moles x Molar mass

We get:

Mass = 0.415 moles x 55.85 g/mol = 23.19 grams

Therefore, the mass of the iron sample is 23.19 grams.

### Problem 5:

How many molecules are present in 6.00 moles of carbon dioxide (CO2)?

**Solution:**
To find the number of molecules, we first need to convert moles to molecules using Avogadro’s number. One mole of any substance contains 6.022 x 10^23 molecules. Using this conversion factor, we calculate:

Number of molecules = Number of moles x Avogadro’s number

Plugging in the values, we get:

Number of molecules = 6.00 moles x 6.022 x 10^23 molecules/mol = 3.61 x 10^24 molecules

Therefore, there are 3.61 x 10^24 molecules of carbon dioxide in 6.00 moles.

### Conclusion

Mole calculations are fundamental in chemistry and are used to determine the amount of a substance in moles. By practicing various mole calculation problems and using this answer key as a guide, you can improve your understanding of the concept. Remember to pay attention to units and conversion factors when solving mole calculations to ensure accurate results.