1. What is Number-Average Molecular Weight?

Definition: The Number-Average Molecular Weight is a measuring system that requires counting the total number of molecules in a unit mass of polymer irrespective of their shape or size.

Purpose: It helps polymer scientists and engineers understand the average size of polymer molecules in a sample.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( M_n \) — Number-average molecular weight (kg/mol)
• \( m_{repeating} \) — Molecular weight of repeating unit (kg)
• \( p \) — Probability of finding repeating unit AB

Explanation: The molecular weight of the repeating unit is divided by (1 - probability) to account for the distribution of polymer chain lengths.

3. Importance of Number-Average Molecular Weight

Details: This measurement is crucial for determining polymer properties like viscosity, mechanical strength, and thermal behavior.

4. Using the Calculator

Tips: Enter the molecular weight of the repeating unit in kg and the probability of finding repeating unit AB (between 0 and 0.99). All values must be valid.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical range for molecular weight of repeating units?
A: For most synthetic polymers, repeating units typically range from 0.01 to 0.2 kg/mol.

Q2: How is the probability of finding repeating unit AB determined?
A: This is typically determined experimentally or calculated from reaction kinetics in polymerization processes.

Q3: What's the difference between number-average and weight-average molecular weight?
A: Number-average considers all molecules equally, while weight-average gives more weight to larger molecules.

Q4: Can this calculator be used for copolymers?
A: Yes, but the repeating unit molecular weight should be the average of the different monomer units.

Q5: What if my probability is 1 or greater?
A: The probability must be less than 1 for the calculation to be valid, as division by zero would occur.