Study Guidance for GOB Chemistry: Sampling, Mass, Moles, and Analytical Methods

Q377. What is the relationship between mass and moles? What is the volume of 10 grams of N2 at normal pressure and temperature (NTP)? What is the number of molecules does it contain?

Background

Topic: Mole Concept and Gas Laws

This question tests your understanding of the relationship between mass, moles, and the number of molecules, as well as the calculation of gas volume at standard conditions.

Key Terms and Formulas

• Mole: A unit representing $6.022 \times 10^{23}$ particles (Avogadro's number).

• Molar mass of N2: 28 g/mol

• Volume of 1 mole of gas at NTP: 24.45 L

• Number of molecules: $\text{moles} \times 6.022 \times 10^{23}$

Step-by-Step Guidance

1. Calculate the number of moles of N2 in 10 grams using the molar mass: $\text{moles} = \frac{\text{mass}}{\text{molar mass}}$.

2. Determine the volume occupied by these moles at NTP using: $\text{volume} = \text{moles} \times 24.45$ L.

3. Calculate the number of molecules using Avogadro's number: $\text{number of molecules} = \text{moles} \times 6.022 \times 10^{23}$.

4. Set up the calculations for each step, but stop before plugging in the final values.

Try solving on your own before revealing the answer!

Final Answer:

Number of moles: $\frac{10}{28} = 0.357$ mol

Volume at NTP: $0.357 \times 24.45 = 8.73$ L

Number of molecules: $0.357 \times 6.022 \times 10^{23} = 2.15 \times 10^{23}$ molecules

We used the molar mass, gas volume at NTP, and Avogadro's number to find the answers.

Q378. How many molecules of water do you drink if you consume 250 ml (or grams) of water?

Background

Topic: Mole Concept and Avogadro's Number

This question tests your ability to convert mass to moles and then to the number of molecules using Avogadro's number.

Key Terms and Formulas

• Molar mass of water (H2O): 18 g/mol

• Avogadro's number: $6.022 \times 10^{23}$ molecules/mol

• Number of molecules: $\text{moles} \times 6.022 \times 10^{23}$

Step-by-Step Guidance

1. Convert 250 ml of water to grams (since density of water is 1 g/ml, 250 ml = 250 g).

2. Calculate the number of moles: $\text{moles} = \frac{250}{18}$.

3. Multiply the number of moles by Avogadro's number to get the number of molecules.

4. Set up the calculation for the number of molecules, but stop before the final multiplication.

Try solving on your own before revealing the answer!

Final Answer:

Number of moles: $\frac{250}{18} = 13.89$ mol

Number of molecules: $13.89 \times 6.022 \times 10^{23} = 8.36 \times 10^{24}$ molecules

By converting mass to moles and multiplying by Avogadro's number, you find the total number of water molecules consumed.

Q379. What is the difference between polydisperse and monodisperse aerosols?

Background

Topic: Aerosol Science

This question tests your understanding of particle size distribution in aerosols, which is important in environmental and occupational chemistry.

Key Terms

• Polydisperse: Aerosols with a wide range of particle sizes.

• Monodisperse: Aerosols with particles of nearly uniform size.

Step-by-Step Guidance

1. Define polydisperse aerosols and explain their characteristics.

2. Define monodisperse aerosols and explain their characteristics.

3. Discuss why understanding the difference is important for sampling and analysis.

4. Set up a comparison table or summary, but stop before giving the full explanation.

Try solving on your own before revealing the answer!

Final Answer:

Polydisperse aerosols contain particles of various sizes, while monodisperse aerosols have particles of nearly the same size. This affects how aerosols are sampled and analyzed in environmental and occupational settings.

Q380. What is gravimetric analysis?

Background

Topic: Analytical Chemistry

This question tests your understanding of gravimetric analysis, a fundamental technique for quantifying substances based on mass.

Key Terms

• Gravimetric analysis: Analytical method based on measuring mass.

• Filter collection: Used to collect particulates for weighing.

Step-by-Step Guidance

1. Explain the principle of gravimetric analysis: measuring mass before and after collection.

2. Describe how samples are collected (e.g., using filters).

3. Discuss how the mass difference is used to determine the amount of analyte.

4. Set up the calculation or process, but stop before giving the final result.

Try solving on your own before revealing the answer!

Final Answer:

Gravimetric analysis involves collecting a sample, weighing the filter before and after collection, and determining the mass of the analyte by the difference. This method is highly accurate for quantifying particulates.

Q381. How is airborne concentration calculated from filter mass?

Background

Topic: Environmental Chemistry and Analytical Methods

This question tests your ability to calculate airborne concentration using the mass collected on a filter and the volume of air sampled.

Key Terms and Formulas

• Airborne concentration: $\frac{\text{mass collected}}{\text{volume of air sampled}}$

• Units: $\mu g/m^3$ or $mg/m^3$

Step-by-Step Guidance

1. Identify the mass collected on the filter (e.g., in micrograms or milligrams).

2. Determine the total volume of air sampled (e.g., in cubic meters).

3. Set up the formula: $\text{concentration} = \frac{\text{mass}}{\text{volume}}$.

4. Prepare to plug in the values, but stop before calculating the final concentration.

Try solving on your own before revealing the answer!

Final Answer:

Airborne concentration: $\frac{50 \mu g}{0.720 m^3} = 69.4 \mu g/m^3$

By dividing the mass collected by the volume of air sampled, you obtain the airborne concentration.